a. En tu escritorio (translates to "in your desk")
REASON: answer choice B is saying "in the door" which makes no sense. Answer choice C is saying "in the window" which also makes no sense, and answer choice D is saying "in your watch". Since answer choices B,C, and D make no sense the answer has to be A which is the answer that makes the most sense.
I'm in Spanish 2 :)