The correct option is A scattering of light
The molecules of air and other fine particles in the atmosphere have a size smaller than the wavelength of visible light. These are more effective in scattering light of shorter wavelengths at the blue end than the light of longer wavelengths at the red end. The red light has a wavelength of about 1.8 times greater than blue light. Thus, when sunlight passes through the
atmosphere, the fine particles in the air scatter the blue colour (shorter wavelengths) more strongly than red. The scattered blue light enters
our eyes. So, the scattering of light is responsible for the blue colour of the sky.