Choose the most suitable option to replace the highlighted part of the sentence:
No sooner had I arrived at the platform when the train left.

The correct option to replace the highlighted part of the sentence is "than the train left".

Grammar Rule & Logical Breakdown:

1. Correlative Conjunction Pair: The adverbial phrase "No sooner" is standardly paired with the conjunction "than", not "when" or "then". The correct structure is always:
No sooner ... than ...

2. Inversion and Tense Consistency: When a sentence begins with a negative adverbial like "No sooner", inverted word order is used in the main clause (auxiliary verb before subject: "had I arrived"). The clause introduced by "than" takes the simple past tense ("the train left") to indicate the event that occurred immediately after.

3. Option Evaluation:

• "than the train left" (Correct): Properly pairs "No sooner" with "than" and uses the correct simple past tense verb phrase.

• "then the train left" (Incorrect): "Then" cannot be used as the second part of the "No sooner" conjunction pair.

• "when the train had left" (Incorrect): "When" is paired with "Hardly" or "Scarcely", not "No sooner".

• "than did the train leave" (Incorrect): Inversion is only required in the initial clause after "No sooner" (which is already inverted: "had I arrived"). The clause following "than" follows standard subject-verb order.

Corrected Sentence:
"No sooner had I arrived at the platform than the train left."