which is shown by the letter x?\n○ trough\n○ amplitude\n○ crest\n○ wavelength

Brief Explanations:

In a wave, the trough is the lowest - point, the amplitude is the distance from the equilibrium to a crest or trough, the crest is the highest - point, and the wavelength is the distance between two consecutive identical points (like two crests or two troughs). The letter X in the wave diagram represents the distance between two consecutive crests, which is the wavelength.

Answer:

D. wavelength