Sunset is red because at that time the light coming from the sun has to travel :
(a) lesser thickness of earth's atmosphere (b) greater thickness of earth's atmosphere
(c) varying thickness of earth's atmosphere (d) along the horizon
During sunrise and sunset, the rays have to travel a larger part of the atmosphere because they are very close to the horizon. Therefore, light other than red is mostly scattered away. Most of the red light, which is the least scattered, enters our eyes. Hence, the sun and the sky appear red.