Concept:"Who" is the correct pronoun to use when referring to people as the subject or subject complement of a clause.
Explanation:The sentence requires a pronoun that asks for the identity of the strange guests, and guests are people.
"Who" is used to refer to people in the subject position of a question.
"Whom" is the object form of the pronoun and would only be correct if the people received the action, which is not the case here.
"Which" is normally used for things or when making a choice from a known group, so it does not fit this context.
"What" is used for objects, ideas, or general unknown information, not for identifying people.
Since the speaker is asking about the identity of human beings, "who" is the most appropriate and grammatically correct option.
Answer:C. who