The correct option is d: sunlight is travelling through less air at noon than at sunset.
-The light from the Sun is really made up of all the colors of the rainbow.Blue light waves are shorter than light waves of any other color. Sunlight is scattered in all directions by the tiny molecules of air in Earth's atmosphere. Blue is scattered more than other colors because it travels as shorter, smaller waves.
-When the Sun is lower in the sky, sunlight goes through more of the atmosphere. The blue light is scattered so much that the reds and yellows pass through to your eyes.
A is not true; dust is present during the day and sunset too. Dust, pollution, and haze in the daytime can make the sky look grayish or even white, and sunsets look less colorful.
B is not true, sun´s heat does not "clean the air".
C is false, at sunset, sunlight goes through more of the air (or atomosphere) not at noon