Which interrogative pronoun best completes the sentence? how is the pronoun used? to will you be sending the invitations? a. who; subject b. whom; object of a preposition c. whose; possessive d. whom; direct object

The answer is d., to whom will you be sending the invitations?

To WHOM. 

It is used in situations where other pronouns is too informal.

It also helps to remember the answer to that question. You wouldn't say "I'm sending an invitation to he", would you? You would say "I'm sending an invitation to him" , therefore, because of the "him" you would use "whom" in the question. 

"Who" would be used in a question such as "who is that?" as the answer would be something like "He's the one with the glasses".

Of course this works with she and her, but he and him is easier to explain with.