Answer:
B: The word "very" is an adverb that tells to what extent.
Explanation:
The word "very" in the sentence presented is an adverb of degree with the function of modifying the adjective tall by emphasizing the extent (how tall) . Adverbs of degree modify an adjective or another adverb. The word very can also be an adjective in other cases but it is used to emphasize a noun : this very minute (=now).