Question

Which interrogative pronoun best completes the sentence?

Answer 1

Out of all the pronouns, "whom" makes the most sense grammatically. This allows us to narrow it down to A or D.

In order for it to be the object of a preposition, there would have to be a preposition in front of it. "For," however is not a preposition. In fact, there isn't a preposition in the whole question.

This leaves us with option D, which is the correct answer.

You can better tell it's a direct object if you flip the sentence around a bit.
Your sister is waiting for whom?
"Whom" is essentially receiving the action. "Whom" is what your sister is waiting for.

Answer: D. whom; direct object.