The train __________ before we reached the station.

The correct answer is B: had left. This sentence describes two past actions, where one action happened before the other. To correctly express this sequence of events, we use the past perfect tense for the action that occurred first, and the past simple tense for the action that occurred second. In this sentence, 'the train leaving' happened before 'we reached the station'. Therefore, 'had left' (past perfect of 'leave') is used for the earlier action, and 'reached' (past simple of 'reach') is used for the later action. The past perfect is formed with 'had' + the past participle of the main verb. Options A ('left', simple past), C ('leaves', simple present), and D ('has left', present perfect) are incorrect because they do not accurately convey the sequence of two completed actions in the past, where one preceded the other.