Scarcely had he finished his speech __________ the audience started clapping.

The correct answer is A: when. This sentence employs a specific correlative conjunction structure used with adverbs like "Scarcely" or "Hardly" when they begin a sentence. This structure emphasizes that one event happened immediately after another.

The fixed grammatical pattern is: Scarcely/Hardly + had + subject + past participle + when + past simple.

• The first part of the sentence, "Scarcely had he finished his speech," correctly uses the inverted past perfect structure (Scarcely + had + subject + past participle).
• The second part, which describes the immediate subsequent event, must be introduced by the conjunction "when."

Therefore, "Scarcely had he finished his speech when the audience started clapping" is the grammatically correct construction.

Let's examine why the other options are incorrect:

• B: than is used with "No sooner" (e.g., "No sooner had he finished his speech than the audience started clapping.").
• C: then is an adverb of time, not a conjunction used in this specific correlative structure.
• D: where is an adverb of place and is entirely inappropriate in this context.

Mastering these specific correlative conjunction pairs is essential for accurate and sophisticated English usage.