Adverbs typically answer questions like "how" and most commonly end in -ly (but don't be fooled, there's many, many more.)
In this sentence, "forcefully" is telling you HOW the soccer player kicked the ball. A is an adjective, B is a noun, C is a verb, and D is also a noun. Adverbs modify verbs, and since "forcefully" modifies "kicked," the answer is C.
