The correct option is A fast
Adverbs modify verbs, adjectives and sometimes other adverbs in a sentence.
In this sentence, the adverb 'very' modifies the adverb 'fast'. 'Very' tries to measure the speed at which she is running. Hence, Option C is correct.
Option A is incorrect because the verb 'is running' is not being modified by the adverb 'very' in this sentence.
Option B is incorrect as the adverb 'very' acts as the modifier in this sentence.
Option D is incorrect as adverbs don't modify pronouns.