Here are the correct pronouns for each question:
Question 20:
The student who spent three weeks on the essay made an A.
Answer: A. who
Question 21:
To whom should I send the last invitation?
Answer: B. whom
Question 22:
I can't figure out who left the note on my desk.
Answer: A. who
Question 23:
My mailman, whom I have known for twenty years, retired last week.
Answer: B. whom
So the answers are:
20 - A
21 - B
22 - A
23 - B