A wavelength is the length of a complete wave cycle, from one crest to the next crest or one trough to the next trough.

Point A is the length from one crest to the next crest; thus, point A represents a wavelength.

The crest of a wave is the upper most point that a wave rises up. Point C is at the upper most point of a wave.

The trough of a wave is the lowest most point that a wave falls downward. Point B is at the lowest most point of a wave.

The amplitude is the distance from rest to a crest or from rest to a trough. Point D represents the distance from rest to the trough; thus, point D is the correct answer.