a. It's not informing you of anything as this is an opinion.
c. It's not describing vanilla ice cream, its just explaining why they like it.
d. It's not made to entertain as it has no real plot, it's an opinion
b. Persuade is the only reasonable answer as the author is trying to tell you why vanilla ice cream is the best, they are trying to persuade you to either eat vanilla ice cream or make it your favorite.
