The correct answer is C. A pronoun has the same function as a noun, within a sentence. So, the pronoun usually refers to people, things, or something that you can mention. On the other hand, prepositions indicate meanings such as place, time, reason, relationship, among others. They do not refer to things or people like pronouns and nouns do. In fact, pronouns can be the object, that is complement, of the preposition.
