Identify the word which the adverb modifies:
You are walking too slowly.
The correct option is A slowly
Adverbs modify verbs, adjectives and sometimes other adverbs in a sentence.
In this sentence, the adverb 'too' modifies the adverb 'slowly'. It states the extent to which the person is walking slow. Hence, option A is correct.
Option B is incorrect as adverbs don't modify pronouns.
Option C is incorrect as 'too' is the modifying adverb in this sentence.
Option D is obviously incorrect.