Based on all the information I definitely know when filling in the blanks I would argue the test is at worst wrong, at best poor. I picked 'a road' naturally and tried to justify 'the road' but couldn't in the context of the passage...

Reading the passage back as if I had received it in an e-mail or over the phone (i.e. I wasn't there or aware of the scene) either version would be fine however;

'the road' could imply:

• a singular road view-able from the window
• the (obviously) 'main' road view-able from the window
• simply the road the man happened to be crossing
• any paved pathway for cars known idiomatically as 'the road'

'a road' could imply:

• I was being given minimal/pertinent/formal information

• the writer was simply creating an image/setting a scene for me (which the rest of the passage certainly does)

Based on the language used until that point in the passage (and especially knowing it was a teaching aide) I would immediately question usage of 'the road' as prior knowledge of any road had not yet been established.

This would change if for example you were at this university standing next to the person as they described their surrounds and you yourself were able to clearly see out of the window (and it wouldn't matter if there was one road or many, 'the road' would make sense as you could see the young man and 'the road' in this context is the road he is on).

However, when being asked to fill in blanks, as we are here, we have to be as un-assuming and un-restrictive as possible and because of this I feel 'a road' makes more technical sense as it implies less specifics and doesn't used idioms (Although part of learning a language is learning related idioms and colloquialisms).

An interesting example where this changes: imagine the young man was in-fact crossing a bridge - I suspect 'the bridge' would be more apparently wrong to more people as we are not aware of any bridge in the scene at this moment in time. Bridges are not idiomatically known as 'the bridge' without more situational context but roads due to their ubiquity are commonly known 'the road'.

n.b. Are nearly all roads connected, at least within a country, to the extent that there is a singular road? The road?

Unfortunately, by the time I read the passage I already knew the "right" answer, so it was hard to forget it.

I ran this passage by four other members of my family. Two said "the road" without hesitating, one said "a, er, the road" and the fourth said "a road".

the test's answer is unequivocally correct

I can't quite agree with that. If this office in the university happened to open upon an intersection, or was high and had a view of a number of roads, I think you might say "crossing a road". In fact, if this was some third or fourth-floor office, to say "the road" might sound strange.

I must admit I tend towards "the road", but I think the test is worded poorly. In this case it is more of a colloquialism than strict usage.

For example, if it said "The young man picked up a dog" we would (hopefully) agree that the use of "a" was correct, since the dog hadn't been previously mentioned. To argue that it is "the road" is asking for a more sophisticated knowledge of English usage, than simply teaching the use of a/an/the.

You could almost argue that the test-setter (setting-person / examiner) unconsciously wrote "the road" drawing on their knowledge of things like "why did the chicken cross the road" (rather than "a road") and not even realize the question is ambiguous.

I think the test is incorrect. Either "a" or "the" is valid.

In general, we use "the" when we are talking about a specific thing, and "a" when we are talking about any instance of a thing. That is, we use "the" when we have previously identified the thing that we are talking about, or when there is only one thing that we could, in context, be talking about.

If we had previously said that there was only one road in town, or only one road visible from this window, or if we had identified the road that the young man was standing beside, etc, then we should refer to it as "the road". But in this paragraph, we have not done anything like that. There was no mention of a road before this. So I think that arguably the correct answer is "a road".

To say, "it's THE road because obviously we're referring to the one road that he is crossing" is unconvincing, because by that reasoning, we would never use the word "a". "I am sitting in AN office ..." But I am sitting in one specific office, the one right here that I am sitting in. But the correct answer is "an" and not "the" because we have not previously identified the office, and there are many possible offices that I could be sitting in. Etc.

That said, many, perhaps most, English speakers would say "the road". Frankly, I'm hard-pressed to say why. I don't think there's a good grammatical or logical reason for it. Even in the vaguest references, people often say "the road". Like it's common for parents to tell young children, "Always look both ways before crossing the street." You almost always hear this as "the street", not "a street", even though the rule presumably applies to any street and not one particular street somewhere. (Like, you should look both ways before crossing Maple Street, but feel free to run out into Broad Street without looking.)

The first thing to understand is that the test's answer is unequivocally correct. The young man in the text crosses the road, not a road: if you ask a hundred native speakers to fill that box, you'll get at least 99 the's.

Second, (and this is edited in later), that judgment isn't universal in its application: there are plenty of cases where it's correct to say, crossing a road.

But why that's so is tricky. I'll try in this answer to give an ad-hoc explanation, and maybe other users can refine or elaborate on it, in the comments or in new answers.

One thing I want to get out of the way early on is this: it's not because there's only one possible road. Even if there are two roads, and it's not clear which one the man crosses, he's still crossing the street:

Peter stood at the intersection and waited for the light. When it changed, the hurried across the street and ducked into a cafe.

I think the road or the street or the highway is a broad, generic area, like the outdoors or the countryside or the ocean. In addition to discrete roads and highways- and oceans- there's the collective road and highway and ocean, which always takes the.

Columbus sailed across the ocean; vikings pillaged the countryside; the chicken crossed the road.

Actually this is kinda weird. My first gut feeling was that "A man is crossing the road" is obviously correct. But then i thought that "A man is crossing a road" does'nt really sound wrong either. Let's compare the following

The man crosses the road. Then he goes to the library. There he sits down on a chair

with

The man crosses the road. Then he goes to a library. There he sits down on the chair.

Probably most people would consider the first paragraph correct and the second one incorrect. I think it depends on the implied context, naturally one assumes there is only one library in town (or in that part of town) and many chairs in the library. But assuming there are a lot of very small libraries around, and the one the man visits has only one chair, the second paragraph would be correct.

Similarly the following also sound correct.

The man walks to an intersection. Then he walks across a street.

I think the implication of using "a" vs "the" depends on the context. Are there many things around or only one. And to get back to the original example, there usually is only one street around (unless you are at an intersection).

The young man is crossing the road.

We are looking at the situation from the young man's perspective. This sentence describes a situation in which he is crossing a road. There is only one road that he is crossing.

We might say

The young man is about to cross the road.

even when the young man is at a crossroads and about to cross one of the roads meeting there. It implies that either we know or the young man knows which of the roads is about to be crossed. It might even imply that we simply don't care which one will be crossed.

The young man is crossing a road.

This is a perfectly correct sentence, but it describes a different situation. It is not any different from the young man's perspective, but it is different from the speaker's perspective: apparently, the speaker hasn't been following what the young man has been doing, and now catches up with him; and apparently, the speaker is aware of multiple roads, and wants to express that the young man is crossing one of them.

Similarly, I will say

From my chair, I can touch the wall with my fingers.

even though there are four walls in my office. It is because I can only touch the nearest one, or because I don't care to point out that there are actually multiple walls in my office. I might even say this when I can actually touch more than one wall. In that case, I'm using "the wall" to refer to all of the walls collectively.

Similarly, "the road" may refer to all roads collectively.

In most contexts, "crossing the road" is better, purely because it's an idiomatic phrase. For example, somebody might very well begin a conversation with "On my way here, I was nearly hit by a car while I was crossing the road," in a context where there's no information about which car or which road. They'd say "hit by a car" because the car isn't uniquely defined (compare, "hit by the mayor's car"). The road also isn't uniquely defined so you'd expect "crossing a road" (compare "entering a shop") but we usually say "crossing the road" because... well, that's what we usually say.

One could also argue that, in the context of looking out of a particular window (as in the text linked from the question), it's quite likely that there's only one road the man could be crossing. In that case, one would be justified in saying "crossing the road" without needing to appeal to it being idiomatic. Similarly, you might say "I looked out of my window and saw somebody entering the shop" if there's only one shop you can see from that window.

However, "crossing a road" is not wrong and there are situations in which it might even be better, especially in situations where "a" could be replaced by "any". For example, "Did you have to cross a/any road to get here?" In other situations, it works equally well. One might argue that there's a slight difference in nuance between "I saw a man crossing a road in the distance" and "I saw a man crossing the road in the distance" but they're essentially the same. The first one suggests "I saw a road in the distance [i.e., far away]. There was a man walking from one side of it to the other." The second suggests, "I saw a man who was walking from one side of a road to the other. The road he was on was far away."

While others have focused on general idiomatic usage, let me make an argument specific to this context for why "a" is perfectly reasonable.

The whole paragraph, up until that point, had a consistent flow to it: introduce something vaguely (almost in a dream-like way) using "a," and then hone in on it using "the." For instance:

Next to the [previously introduced] desk there is a window. Looking out of the window [that we just introduced] I can see a young man. He is wearing a uniform [which I will soon describe] ...

With that flow, I would expect "a road," and that the next sentence would say something about the road. Given that we don't even know where in town this young man is, I don't see how we could expect to know what road he's crossing, and so I would stick with "a". At this point, the road is as vague a as everything else was when we were first introduced to it.

It's interesting to note that the next sentence is not about the road, which is the first time in the paragraph that the pattern was broken. So it's really a question of when the author wants to break that pattern, and how forcefully. They intended to break it with the article "the," snapping us away from the dream-like description into a specific time and place; my style would have been to be more subtle about it, by keeping the article and simply changing the content (but not linguistic style) of the sentence. Neither approach is wrong, and they have different meanings (if at a very subtle level).

From the context (looking out a window), presumably only one road is visible from the window. If multiple roads are visible, then "a road" might be appropriate. "A road" usually sounds odd in this context.

I think the answer here is that you're wrong because standard English grammar is wrong. Two wrongs make a right in this case.

Strictly speaking, it should be "a road" as you suspect, because the road hasn't been announced before, and there's nothing special about this particular road. Just like we say he's walking towards "a car", rather than saying "the" car, despite the fact that he's probably walking to the very specific car he has keys for.

However, in common usage, we just don't do it that way. Even in a sentence where we're clearly talking about a non-specific road, like "which way should I look first before crossing the road in England?", we use "the" over "a".

I'm going to say the answer is "because that's the way it's done". I wouldn't try to over-analyze it past that, because there's no logical answer that I can see.

In trying to understand why "the road" is the accepted and common usage in this sentence, we must look at the adoption of the term "road". It was incorporated into the language in the same standing as a geographic feature. "Little Red Riding Hood crossed the river, went through the woods, around the mountain, over the hill, across the plains, and found herself in the desert." (Non-standard version of the tale) So, as roads became features of the landscape, their usage became aligned with that usage.

As a side note, it's interesting to think that "the road" is common when compared to the common usage of "a path".

Usually, we say "I am crossing THE road"

However if you say "I am crossing A road", it has a different meaning. It all depends on the context in which you're using the sentence.

If I were to say "I am crossing a road", it would usually mean that you're crossing an unknown road or a road that you don't know the name of. In any other context you should say "I am crossing the road."

The rule is: THE means a specific thing. Ex. the dog, the chair, the house. A means something in general.

If I were to say "I see a dog", it would mean I was just walking down the street and saw some unknown dog. Whereas if I were to say "I see the dog", it would mean I see a specific dog. For example I am looking for my neighbor's dog which ran away and then I see it.

I hope that makes sense :)