Answer: A. wavelength
Explanation:
Wavelength is the distance between the two consecutive crests or troughs.
Frequency is the number of vibrations in one second. It is also defined as the number of crests that pass a point in a given time.
Amplitude is the maximum extent of a vibration or oscillation measured from the position of equilibrium.
Loudness is the perception by an individual of how intense the sound is.
Therefore, the correct answer is wavelength.
