QUESTION
To _____ should this letter be addressed?
ANSWER AND EXPLANATION
Your answer is a. whom
You can just substitute both words into the sentence and see which makes more sense.
To who should this letter be addressed?
Who is normally used when talking to people. In this question, the word who does not make sense.
Try it with whom:
To whom should this letter be addressed?
Whom fits in better than who. Therefore, your answer is a. whom.
<em>If you have any questions, feel free to ask in the comments! :)</em>
