No sooner __________ the bell rung than the students left the class.

The correct answer is "had". This sentence employs the idiomatic expression "No sooner...than," which is a common and formal structure used to indicate that one event happened immediately after another. When "No sooner" begins a sentence, it requires inversion of the subject and the auxiliary verb. This means the auxiliary verb comes before the subject, similar to a question structure. Crucially, "No sooner" is almost always used with the past perfect tense in the first clause (the "no sooner" clause), followed by the simple past tense in the "than" clause. The past perfect tense ("had rung") is essential here because it emphasizes that the action of the bell ringing was completed just before the students left, establishing a clear sequence of events. The complete and correct structure is: "No sooner + auxiliary verb (had) + subject + past participle (rung) + than + simple past (left)." Therefore, "No sooner had the bell rung than the students left the class" is the only grammatically correct construction, effectively conveying the immediate succession of actions.