Choose the sentence that contains a relative pronoun.
a. , put your book bag in your locker.
b. someone is coming down the hill.
c. mr. moon, whom you have met, is my assistant.
d. those are not my shoes.

C. Mr. Moon, whom you have met , is my assistant.

A relative pronoun is a kind of pronoun that is used to attach a phrase to a noun or a pronoun. In the sentence above, the relative pronoun is "whom". Other examples of relative pronouns include that, whichever, who, and which. Below are some examples of sentences that contain relative pronouns.

I have one true love, the one that got away.
She is my mother, the one who gave me everything.
She chose the lavender dress which is more revealing.

Ibelieve it is entry b.