The correct answer is left.

The sentence describes a past event that happened before another past event. The train's departure is the earlier action, while their failure to reach the station is the later one. Therefore, we need a past simple verb tense to accurately depict this sequence.

Let's examine why the other options are incorrect:

• has been leaving: This is the present perfect continuous tense, suggesting an ongoing action that started in the past and may continue into the present. This doesn't fit the context of a single past event.
• leave: This is the base form of the verb, requiring an auxiliary verb to form a proper tense. It's grammatically incorrect in this sentence.
• has leave: This is grammatically incorrect. The correct past participle of "leave" is "left".
• left: This is the past simple tense of the verb "leave," correctly indicating a completed action in the past. This perfectly describes the train's departure before the individuals reached the station.

The past simple tense ("left") accurately reflects the completed action of the train's departure prior to the other action. The sentence's narrative demands a clear past tense to establish the temporal sequence of events. Using "left" establishes the necessary chronology.