The correct options are
C very
D fast
An adverb is a word that modifies an adjective, a verb or another adverb. In the given sentence, the subject "Tom" performs an action "runs" in a manner "very fast". "Very" refers to a degree or extent of something and intensifies the word "fast", which is the manner of speed in which Tom runs. Hence, "very" and "fast" are both adverbs of degree and manner, respectively. "Fast" modifies the verb "run" and "very" modifies the adverb "fast", so options C and D are both correct. Option A is incorrect because "Tom" is a noun, it names a specific person, and is not an adverb. Option B is also incorrect as "runs" is a verb that performs an action, and is modified by an adverb.