No sooner had we reached the bus stop __________ the bus arrived.

The correct answer is C: than. This sentence correctly employs the correlative conjunction "no sooner... than," which is a well-established idiomatic expression in the English language. This specific grammatical structure is utilized to convey that one event occurred immediately following another, emphasizing the close temporal proximity between the two actions. When the phrase "no sooner" initiates a sentence, it is characteristically followed by an inverted subject-verb structure, as seen with "had we reached" instead of the standard "we had reached." Crucially, "no sooner" is invariably paired with "than" to introduce the subsequent event. For instance, a common usage is, "No sooner had I finished my work than the phone rang." Options A ("then"), B ("when"), and D ("where") are incorrect because they do not form the required idiomatic pair with "no sooner." While "when" is often used with similar structures like "hardly" or "scarcely" (e.g., "Hardly had we reached the bus stop when the bus arrived"), it is never used in conjunction with "no sooner." Therefore, "than" is the sole grammatically accurate choice to complete the correlative conjunction "no sooner... than" in this context.