A pronoun is a word that can come in the place of a noun. Option C is correct as it is used to refer to a thing or things belonging to or associated with two or more people or things previously mentioned ( his parents' friends). The other options are wrong as them, their, and themselves is used when it comes in the place of a group of people, a group of people having a connection with something, and a group of people which has a connection with their own selves. The correct answer is C)theirs.