Redness in the atmosphere during sunrise and sunset is caused by:

• Scattering of light is the phenomenon in which a light beam is diverted in all other directions after passing through particles with a size similar to the wavelength of the light employed.
• During the sunset and sunrise hours, the light has to pass through the long patch of the atmosphere. 
• While travelling a long distance, almost all the colours get scattered except red and orange which have comparatively longer wavelengths than the other colours and the size of the particles of the atmosphere. 

Hence scattering of light is the correct response.