Answer: A. who
Explanation:
The secretary _________ is sitting at the first desk on the right can give you the information.
Option A: who is used to refer to the subject of a sentence.
Option B: whom is used to refer to the object of a verb or preposition.
Option C: where is an adverb or conjunction talking about a certain place.
Since the subject of the sentence is being referred to, who is the appropritae choice of word.
