I reached the station, the train ______.

This question tests the understanding of past tenses, specifically when one past action occurred before another. The past perfect tense is used to describe an action completed before another past action or a specific time in the past.

• Option C: "had left" is the correct choice. The past perfect tense (had + past participle) clearly indicates that the train's departure happened before the speaker reached the station, establishing a sequence of past events.
• Option A: "has left" is present perfect, implying a connection to the present, which is inconsistent with the past action of reaching the station.
• Option B: "left" is simple past. While grammatically possible, it doesn't explicitly convey that the train's departure preceded the arrival at the station, which is the intended meaning.
• Option D: "was leaving" is past continuous, suggesting the train was in the process of leaving at the moment the speaker arrived, contradicting the idea that it had already departed.