The correct answer is option 2: atmospheric refraction.

When we see the Sun slightly before it rises and for a short while after it sets, it is due to the phenomenon of atmospheric refraction.

Atmospheric refraction is the bending of light that occurs when it passes through the Earth`s atmosphere. The atmosphere is composed of layers of air with varying densities. When light passes from one medium (such as vacuum or outer space) to another medium with a different density (such as the Earth`s atmosphere), it changes direction.

During sunrise and sunset, the Sun is near the horizon, so the light from the Sun passes through a thicker portion of the Earth`s atmosphere than when it is directly overhead. As a result, the light from the Sun gets refracted or bent as it passes through the various layers of the atmosphere.

This bending of light causes the Sun to appear slightly higher in the sky than its actual position. This phenomenon, known as atmospheric refraction, is the reason why we can see the Sun before it rises and after it sets.

Therefore, option 2, atmospheric refraction, is the correct explanation for why we can see the Sun slightly before it rises and for a short while after it sets.