The answer to this question is a tad nuanced, and that's likely the reason why they're asking it. You'll want to ask if every sock has 1 (and only 1) mate, which is a reasonable assumption. If they say that's a fair assumption to make then the answer is 1/(2n -1) where n is the number of pairs of socks in the drawer. The reasoning here is that given these assumptions, the first sock you pick will not matter. The second sock just has to match the first, and at this point there is 1 match in the drawer, and 2n - 1 socks left in the drawer.
Of course, if they say this is not a reasonable assumption, and provide you with more information on the contents of the drawer (e.g. there are 3 socks without a match, and also a set of three pairs of interchangeable socks, and 3 'normal' pairs) then the question will change considerably, and you'll have to demonstrate a knowledge of probabilities to answer the question properly.