In which of the following sentences is the underlined word an object pronoun?

A. You really should apologize to her.

B. We returned our books to the library.

C. I would like you to listen to me.

D. I gave her a gift for her birthday.

A pronoun is a word that takes the place of a noun. For example:

I, me, he, she, herself, you, it, that, they, each, few, many, who, whoever, whose, someone, everybody, etc.

In the sentences above the right answer lies in the option:

B. We returned our books to the library.

OUR is the pronoun.