Which of the following sentences is grammatically correct?

Correct option is C

The correct sentence is (c).
Explanation: We narrate two past actions where one happened before the other. Use Past Perfect (had + V3) for the earlier action and Simple Past for the later one: “When we reached (Simple Past), the train had already left (Past Perfect).” The adverb already fits naturally before the past participle.
Grammatical rule used:
· Past Perfect shows an action completed before another past action/time point.
· Structure: had + past participle (V3) for the earlier past; V2 for the later past action.
Example: “By the time the movie started, we had found our seats.”
Why other options are incorrect:
· (a) have already left — Present Perfect after a past time clause is wrong.
· (b) has already left — Present Perfect with a past reference (“reached”) is incorrect.
· (d) already left — Simple Past in both clauses can be ambiguous; with “when,” Past Perfect is preferred to show the prior action clearly.