The correct answer is
C.wavelength
In fact, the colour of visible light depends on the wavelength of the wave. The visible light spectrum goes approximately from wavelength of 380 nm (corresponding to violet light) to 750 nm (corresponding to red light), and the different colours in between correspond to different wavelengths. For instance, blue light has wavelength of 450-500 nm, while yellow light has wavelength of 570-590 nm.
