The question is asking us to complete the sentence: "Choose the correct pronoun to complete this sentence: I hate to loan my books to people because they often forget to return ..... First, let's think what we need to put into the gap - it would be a reference to the books - that's what I'm lending to people and what people are not returning. So the books are "they" and in object case they are "them" - the correct option is "them"
