The answer is A.) Amplitude
The Crest is the top of each wave, the trough is the bottom of each wave, and the wavelength is the distance from crest to crest or trough to trough. This leaves only one answer, amplitude which is the maximum amount of vibration or oscillation measured from the equilibrium, so basically the distance from the middle line to the crest.
I hope this helped!! :)) Have a blessed day :D
