The correct option is C She instantly made lunch for everyone.
While reducing a sentence to an adverb/adverb phrase, the subject and
verb is removed as it is connected to another sentence. The resulting
sentence is a simple sentence.
Option B is correct because it contains the adverb form of the sentence, 'instantly' that modifies the verb 'made'. The sentence 'she made it in an instant' is reduced to a single word, 'instantly'. (It can also be reduced to 'in an instant' as a phrase: She made lunch for everyone in an instant.)
Option A is incorrect because the adverb is incorrectly placed, it modifies the verb 'made', hence it should be placed before the verb, and not after it.
Option C is incorrect because the meaning of the sentence is vague; the adverb modifies 'everyone' instead of the verb.
Option D is incorrect because the answer lies in B.