Answer:
D.frequency.
Explanation:
The number of times a wave passes through a particular point in a time of 1 second is called the frequency.
Amplitude is the maximum height the wave reaches from the reference axis of the wave
Wavelength is the distance between the two upper (crest) or lower (troughs) points of a wave.
Crest is the top most part of a wave.
Hence, the question is referring to frequency.