Correct Form: No sooner I had reached the station than the train left:

The correct answer is No sooner had I reached the station than the train left. The phrase 'No sooner' is always followed by an inverted auxiliary verb (had) before the subject (I). The correct structure is: No sooner + had + subject + past participle + than. 'Than' is the correct conjunction to use with 'no sooner' — 'when' is used with 'hardly' or 'scarcely'. Option 2 is incorrect because it lacks inversion. Option 4 incorrectly uses 'when' instead of 'than'. Therefore, the correct sentence is 'No sooner had I reached the station than the train left'.