Answer:
The correct answer is option C. They.
Explanation:
The predicate nominative pronoun helps us to recognize the subject of a sentence, or to refer to it. It can be a single word or a group of words and they will always be connected by a linking verb, that is, a verb that does not demonstrate an action but rather serves to unite the subject with the rest of the sentence. In this case it is the verb are.
All forms of the verb to be are linking verbs.
Given this information we can say that the correct answer is option C.