Answer:
To c. whom do you refer?
Explanation:
Whom is a pronoun that refers to the object of a verb or preposition (like to and from), meaning that this pronoun does not perform the main action, but instead, it receives the action. Therefore, when answering questions with "whom", the answer may be expressed with object pronouns, namely, me, us, you, him, her, its, them.
In the question, whom is the correct option because it is the object of the preposition to, and if we were to answer the question, we would have to express the question with an object pronoun. For example:
Question: To whom do you refer?
Answer: I refer to him.
