Final answer:
The sentence that contains an adverb modifying an adjective is 'A marsh is somewhat similar to a swamp', where 'somewhat' is the adverb modifying the adjective 'similar'.
Step-by-step explanation:
The question asks which sentence contains an adverb modifying an adjective. Among the provided sentences:
A. In a marsh, plants are surrounded by still water
B. A marsh is somewhat similar to a swamp
C. A marsh has meadow plants and grassy vegetation
The sentence that contains an adverb modifying an adjective is sentence B. The word somewhat acts as an adverb modifying the adjective similar, indicating the degree to which a marsh is similar to a swamp.