Which among the following is the actual reason for the apparent flattening of the Sun’s disc at sunrise and sunset?

The apparent flattening of the Sun’s disc at sunrise and sunset is due to atmospheric refraction.

The atmosphere is made up of layers of different optical densities. The deviation of light or other electromagnetic waves from a straight line as it passes through the atmosphere is known as atmospheric refraction. 

Another example of atmospheric refraction is why the Sun is visible to us about 2 minutes before the actual sunrise, and about 2 minutes after the actual sunset.