Final answer:
The guitar and piano playing the same C note at the same intensity differ in timbre, which is the quality that distinguishes their sounds. The correct option is B).
Step-by-step explanation:
If a guitar and a piano both play the same C note at the same intensity, the instruments will have a different timbre. Timbre, also known as the tone quality, is the characteristic of sound that allows us to distinguish between different instruments even if they play the same note at the same loudness.
Unlike pitch or loudness, timbre does not have a direct correlation to a physical quantity like frequency or intensity, making it a more subjective attribute of sound. Terms such as dull, brilliant, warm, and pure are used to describe the timbre of a sound. Option B) is the correct one.