Fill in the blank with the correct form of the verb given in brackets from the options:
The train ________ (leave) before he reached the station.

When two action occurs at different time in the past we use:
Past perfect tense: had + V3 (for action happened first)
Past indefinite tense: V2 (for action happened after the first one)
In the given sentence, there are two actions:
First action: the train leaving (should be in past perfect)
Second action: he reaching the station (should be in past indefinite)
It's clear that 'had left' is the correct form of the verb to complete the sentence.
Therefore, the correct answer is option 2).
Complete sentence: The train had left before he reached the station.