Select the option that will improve the highlighted part of the given sentence. In case no improvement is needed, select 'No improvement required'.
Hardly had he disappeared that her mother clomped down the steps, looking for him.

Correct option is D

The correct substitution of the highlighted words is (d) Hardly had he disappeared when her mother.
Explanation: "Hardly" is a negative adverb that requires inversion and is usually followed by "when" to indicate two closely connected past events.
The structure is: Hardly + had + subject + past participle + when + simple past.
Grammatical rule used: Inversion after negative adverbs (Hardly, Scarcely, No sooner). These require inversion (auxiliary before subject) and correct conjunction (“when” or “before” depending on the word).
Example: Hardly had she left when it started raining.