Indefinite pronouns are pronouns that does not refer to something or someone in particular. It creates a vague image of the subject, object, or complement. In this sentence, there are two pronouns: something, and us. Let's analyze the choices.
(a) Chris is a name of a person. Hence, it is a noun and not a pronoun.
(b) Something is perfect example of an indefinite pronoun. It doesn't clearly answer the question, "What?"
(c) Us is a pronoun. However, it is a personal pronoun in the first person plural case.
(d) That can be a relative pronoun. However, it's used as a demonstrative adjective in the sentence. It implies the location of the book.
