Answer : The correct option is, D because amplitude is the distance between a crest (or trough) and rest.
Explanation :
Amplitude : The measurement of the distance of the wave from the rest position of the wave is known as amplitude. Or we can say that, it is the distance between a crest (or trough) and rest position.
It is generally determined by looking on the graph of a wave and measuring the wave's height from the resting position of a wave.
Thus, from the given graph we conclude that D is the amplitude because it is the distance between a crest (or trough) and rest. While C determine the wavelength of the wave.