Fill in the blanks with appropriate demonstrative or interrogative pronoun.
To ____________ did you give the keys?

The correct answer is: Whom

The correct answer is "Whom".
To whom did you give the keys?
Here, the speaker is asking somebody about the person to whom he gave the keys. To express the question, we must use an interrogative pronoun. Since the speaker is demanding information about the receiver of an action(giving keys to someone), the correct answer is "Whom".