A word that can function as a noun phrase used by itself and that refers either to the participants in the discourse or to someone or something mentioned elsewhere in the discourse is a pronoun. In this sentence, there are two nouns present, which means they are present in a group.
They is used to refer to the subject of a clause. In other words, it usually represents the ‘doers’ of the action described by the verb, and usually refers back to two or more people or things that were mentioned earlier. It is the wrong option.
Its is the positive form of "it". Again "it" is not used in case of groups.
Them is used to refer to the object of a clause. In other words, it usually represents the group of people or things that have ‘experienced’ the action described by the verb, and refers back to two or more people or things that were mentioned earlier