The question is a complex sentence. The main clause is “Attendants of the conference should exit the venue immediately,” ending with an adverb clause “_______ that the fire alarm rings.”
Let’s focus on the adverb clause “_______ that the fire alarm rings.”
Only (B) "in the event that" and (C) "except" are possible answers since both of them can be used before a that-clause to start an adverb clause.
(B) "in the event that" is used to introduce a condition or possibility. In this sentence, it is used to indicate that the attendants should exit the venue if the fire alarm rings. This phrase is appropriate because it conveys the idea of responding to a specific event or situation.
(C) "except" implies that the attendants should exit the venue unless the fire alarm rings, which is the opposite of what is intended.
(A) "despite" should be eliminated because the word “despite” is a preposition and should be directly followed by a noun. "despite the fact that" would have been a correct option.
(D) "because" should be eliminated, too, because when the subordinating conjunction “because” is used to start an adverb clause, the word “that” shouldn’t be used.