Concept:The phrase "no sooner...than" is used to show two events happening immediately one after the other. The conjunction "than" must follow "no sooner".Explanation:In correct usage, "no sooner" begins a sentence, so the auxiliary verb comes before the subject (inversion). For example: No sooner had we arrived than the bell rang. Option A uses "but" — incorrect. Option B uses "and" — incorrect. Option C uses "when" — this works with "hardly", not "no sooner". Option D uses "than" — this is the correct conjunction. Thus, the only grammatically correct sentence is: No sooner had we reached the station than the train left.Answer:Option D: No sooner had we reached the station than the train left.