Select the most appropriate option that can substitute the

Solution

The sentence describes two actions in the past — reaching the station and the train leaving. The train’s departure happened before the other past action, so past perfect tense ("had left") is the correct choice.
"Had left" indicates the train departed before they reached.
"Was leaving" implies both actions happened simultaneously, which is incorrect here.
"Has left" and "is leaving" are present tenses and inappropriate in this past-time context.