Correct option is A
The correct substitution of the highlighted words is (a)
No sooner had she taken a few steps forward than.
Explanation: The expression "No sooner...than" is a correlative pair used for past perfect actions to show that one action happened immediately after another.
It follows the structure:
No sooner + had + subject + past participle + than + clause.
The original sentence incorrectly uses "that" instead of "than" and disturbs the inversion rule.
Grammatical rule used: Inversion with negative adverbials like "No sooner" — when such adverbs begin a sentence, auxiliary verbs precede the subject.
Example:
No sooner had I reached the station than the train left.