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.)