Answer: The option that fixes this comma splice error is A. Because the bell rang, the students left the classroom.
Explanation: The sentence provided contains a comma splice error because two independent clauses ("The bell rang" and "The students left the classroom") have been joined by means of a comma and no conjunction. The best way to correct this punctuation mistake is to transform one of these clauses into a subordinate clause. Option A. is the correct option because it includes a subordinate clause ("Because the bell rang") and an independent clause ("the students left the classroom") and, unlike option B., it creates a cause-and-effect relationship between the two clauses that adds coherence to the sentence.