a. Anyone
An indefinite pronoun is a pronoun (which is a substitution for a noun or noun phrase or "antecedent") that refers to one or more objects, beings, places, ideas, etc. that are not specified. "Anyone" is this type of pronoun because it does not tell us to whom or to what it is referring exactly.
Other examples of indefinite pronouns are any, anybody, anyone, either, neither, nobody, no, someone, some, many, few, and all.