When changing a feeble sound to a loud sound, you increase its amplitude (b).
Explanation:
Loudness is directly related to the amplitude of a sound wave. A larger amplitude means a louder sound.
Frequency: determines the pitch of a sound, not its loudness.
Velocity: is the speed at which the sound wave travels, and it doesn’t affect loudness.
Wavelength: is the distance between two consecutive crests or troughs of a wave, and it’s not directly related to loudness.