Solution:

Concept: The rays are so close to the horizon at sunrise and sunset, they must travel a greater portion of the atmosphere. As a result, light other than red is mostly scattered. Our eyes receive the majority of the red light, which is the least scattered. 

As a result, the sun and the sky appear red.

Because red is the least scattered color, the sun appears red at sunrise and sunset. 

Because sunlight must travel through the greatest amount of atmosphere to reach the observer's eyes at sunset or sunrise. 

As a result, more blue light is scattered from the sun, making the sun appear redder when it rises or sets.