Which statement describes a wave's wavelength?

 The distance between two crests

Explanation:

The wavelength is equivalent to the actual distance that the disturbance (wave) travels over a certain time. It is also defined as the distance traveled by a periodic disturbance that propagates through a medium in a given time interval, for example the distance beteween two crests of the wave.

It should be noted that the wavelength, also known as the spatial period is the inverse of the frequency :

 where is the velocity of the wave