Concept:Relative pronouns connect a clause to the noun it describes, and the correct choice depends on whether the noun refers to a person or a thing and on the pronoun's role in the clause.
Explanation:The sentence describes "strange guests", who are clearly human beings.
The pronoun "which" is used for animals or things, so it does not fit here.
The pronoun "what" also refers to things and cannot replace a person.
Both "who" and "whom" do refer to people, so the choice must be made based on grammatical function.
"Who" acts as the subject of a clause and performs the verb.
"Whom" acts as the object and receives the action of the verb or a preposition.
Because the strange guests are the ones carrying out the action in the clause, they are the subject.
A subject pronoun is therefore required.
The only option that correctly serves as a subject pronoun for people is "who".
Choosing "whom" would be incorrect because no verb or preposition directly governs the guests as an object.
Thus, the sentence reads smoothly and correctly with "who".
Answer:B. who