During sunrise and sunset, the sun appears reddish-orange because of a phenomenon called Rayleigh scattering. Option 1 is incorrect because the sun emits light of various wavelengths, not only reddish-orange. Option 2 is also incorrect because the atmosphere does not absorb all other colors except for reddish-orange.

Rayleigh scattering occurs when the Earth`s atmosphere scatters the shorter wavelengths of light (blue and violet) more than the longer wavelengths (red and orange). This scattering happens because the molecules in the atmosphere are much smaller than the wavelength of visible light. As a result, when the sun is at a lower angle during sunrise or sunset, its light passes through a thicker layer of the Earth`s atmosphere. Due to Rayleigh scattering, the shorter blue and violet wavelengths are scattered multiple times, while the longer red and orange wavelengths are less scattered and continue to reach our eyes. This selective scattering of colors gives the sun a reddish-orange appearance.

Therefore, option 3 is correct because reddish-orange light is least scattered by the atmosphere compared to the other colors.