The correct answer is C. Books
Explanation:
A pronoun antecedent refers to a word that refers to a pronoun previously mentioned, this means the antecedent and the noun it refers to are connected in meaning and number, that is if the antecedent is plural the pronoun should be equally plural. In the case of the sentence "Those are the books that need to be returned to the library", "those" is the main pronoun of the sentence as this word is directly link to the action and the pronoun antecedent is "books" as this word refers to the pronoun "those", this means when the person says the word "those" he or she is referring to the books that are later mention, also, because "books" is the antecedent of those both words refer to plural.
