The wave with the longest wavelength is the wave B. The distance between the troughs is the largest shown.
What is the wavelength of a wave ?
The distance between two successive points that are in phase—that is, at the same place in their oscillation cycle—is known as a wave's wavelength. Stated differently, wave length is the separation between two successive wave peaks (crests) or troughs. Traditionally, wavelength is represented by the Greek letter lambda (λ).
Wavelength is inversely proportional to the frequency of a wave, meaning that as the frequency increases, the wavelength decreases, and vice versa.