He large, flat piece of wood within a piano serves the same purpose as A. a bamboo reed. B. the hollow box within a violin. C. a set of wooden drum sticks. D. a guitar pick.

The correct answer is B. the hollow box within a violin.
This large, flat piece of wood found in a piano produces the sound when the piano is played, the same way the hollow box produces the sound coming from a violin. The two items have the same purpose, which is to provide the instrument with sound.