Absolutely right, A is colloquial shorthand and not grammatically proper. Adding who improves the text but doesn't fix it; there's still an unsemantic tense use. How can you close the curtains if you've already left the office? Both B and C are acceptable and correct remedies.

B indicates you should, as part of the process of leaving, close the curtains.

C specifies that the curtains are to be closed immediately before exiting. The use of present tense tells us that the two actions happen at (effectively) the same time, and logic tells us that the curtains must be closed before you've left. Whether or not closing the curtains is part of the process of leaving is technically ambiguous but irrelevant.

(If you are the last person to leave applies in both cases, obviously.)

You're also spot on with the addition of the; it should indeed read the curtains. Technically speaking, without the it's unspecified which curtains should be closed, though the meaning is readily evident without the article.

Bonus alternative: you might also fix the sentence like this:

Please close the curtains if you are the last one left in the office.

Here, one need not be leaving to be obligated to close the curtains.

I like option "b)." Also, "Please close the curtains if you are the last one to leave the office" sounds natural.

A) It's not really good English. It's more like a shorthand thing. Some proper ways to say it are "Please close the curtains if you are the last one left in the office." or "Please close the curtains if you are the last one in the office." or "Please close the curtains if you are the last one to leave the office.".(If you didn't notice, it is the curtains). B and C are also correct ways of saying it, and all of these mean the same thing. A) is just a quick way of saying it, and it isn't proper grammar.