The distance between two crests describes a wave's wavelength.
Wavelength is the distance between identical points in the adjacent cycles of a waveform signal propagated in space or along a wire, as shown in the illustration. In wireless systems, this length is usually specified in meters,centimeters, or millimeters.
The correct answer between all the choices given is the third choice or letter C. I am hoping that this answer has satisfied your query and it will be able to help you in your endeavor, and if you would like, feel free to ask another question.