Final answer:
A base fiddle is louder than a harp primarily because of its larger sounding board, which efficiently amplifies the sound, though thicker strings and lower pitch also contribute to its loudness.
Step-by-step explanation:
A base fiddle, or double bass, is louder than a harp for several reasons, but among the options provided, option B) sounding board is the most significant. The thicker strings of a double bass (option A) do contribute to its volume by moving more air when vibrated, and the lower pitch (option C) also plays a role in the sound's audibility. However, the primary reason for the difference in loudness lies in the sounding board, which is responsible for amplifying the vibrations from the strings. In the case of the double bass, the large sounding board acts as a resonator that efficiently transmits the energy from the vibrating strings into the surrounding air, creating a louder sound. In contrast, while a harp has a sounding board, it is typically smaller in comparison to that of a double bass, and so doesn't amplify the sound as much.