B
Explanation:
The speed of a wave depends on the medium, and the frequency of the wave depends on the source.
Here, the source of the travelling wave is the vibration of the speaker up and down, and the medium of that wave is the string.
Statement I is correct. A decrease in the frequency of the sound from the speaker results in a decrease in the frequency of the travelling wave in the string. The wave speed is constant, as the medium of the travelling wave remains the string. The wavelength is inversely proportional to the frequency according to the wave equation:
v=λf→λ=v/f
As a result, the wavelength must increase.
Statement II is also correct. A louder sound means that the amplitude of the longitudinal sound wave in the air is larger. This is caused by greater amplitude vibrations from the speaker, which also produces larger amplitude vibrations in the string.
Statement III is incorrect. As previously mentioned, the wave speed is constant as the medium remains the same. Even though the frequency of the wave has increased, the wavelength will decrease in proportion to keep the wave speed equal to the wavelength times the frequency.
Therefore, the correct answer is B, I and II only.