When the Sun is near the horizon during the morning or evening, it appears reddish. The phenomenon that is responsible for this observation is

The correct answer is D, scattering of light. Near the horizon, sunlight crosses a much thicker layer of air, and scattering removes most of the blue light before it reaches us.

Tiny gas molecules in the air scatter short wavelengths far more strongly than long ones; by Rayleigh's law, scattering varies inversely with the fourth power of wavelength. At noon the light's path through the atmosphere is short, so the Sun looks white while the scattered blue colours the sky. At sunrise and sunset the path is many times longer, blue and violet are scattered away on the journey, and mostly red and orange light, which has the longest wavelengths, reaches the eye.

A is wrong because reflection only sends light back from a surface and does not change its colour this way. B is wrong because atmospheric refraction makes the Sun visible a little before actual sunrise and after actual sunset, not red. C is wrong because dispersion is the splitting of white light into a spectrum, as in a prism or a rainbow.

Exam tip: blue sky, red sunrise and sunset, and the white colour of clouds are all explained by scattering of light.