Answer:
The answer is: letter A, Observing the interactions of those in love.
Step-by-step explanation:
"Sheer joy" refers to being very happy. For a monster to have "sheer joy," he should observe or experience something that he hasn't encounter before.
So, among the choices above, letter A (Observing the interactions of those in love) is something that the monster will feel very happy to see. This is because, he cannot feel such kind of situation because he is a monster. Many times, people are afraid of him, thus watching people interacting when they're in love would bring sheer joy to the monster.
Thus, this explains the answer.