The definition of wavelength is the distance between two identical points, like the crests of the waves, in a cycle of a waveform, so the distance between two crests, which is the highest point of the wave, this is wavelength
In our question, we have that one crest starts at 1.5 cm and the other starts at 2.5 cm, therefore the wavelength will be 1.0 cm of distance, letter D
