Fill in the blanks with appropriate demonstrative or interrogative pronoun.
With _________________ were you talking?

The correct answer is: Whom

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