Answer:
A. who
Explanation:
In English, we use interrogative pronouns to ask questions. One should know that there are five basic interrogative pronouns: who, what, whom, which and whose.
They are usually easy to identify because they stand in the beginning of the question.
However, the problem may arise when differentiating them from relative pronouns (what, who, which etc.)
Relative pronouns stand in the beginning of relative clauses and connect them to the main sentence. They refer to the noun mentioned in the main sentence and modify it:
That's the guy who stole my wallet.
"who" may seem like an interrogative pronoun, but it refers to "the guy" so it is relative pronoun.
In this case, although both who and what may seem as interrogative pronouns, only who is used to ask a question.
