Fill in the blanks with the most appropriate option:-

No sooner had she finished her speech …… the light went out.

than

The correct option to fill in the blank is than.

The idiom "no sooner had" is used to describe two events that happen very close together, with the second event happening almost immediately after the first. In this sentence, it means that the light went out almost immediately after she finished her speech. The word "than" is used to connect the two events and to show that they happened in quick succession.

The other options are not as appropriate:

• As is used to introduce a clause that explains or describes the first event. In this sentence, it would not be accurate to use "as", as the second event does not explain or describe the first event.
• Then is used to introduce a clause that tells us what happened next. In this sentence, it would not be accurate to use "then", as the second event happened at the same time as the first event.
• No improvement is not correct, as the sentence needs a word to connect the two events.