Explanation:
The inhomogeneous refractive index of Earth's atmosphere is responsible for the oval shape of the evening Sun.

Refraction of Light:
Refraction is the bending of light as it passes from one medium to another with a different refractive index. The refractive index of a medium is a measure of how much the speed of light is reduced when it passes through that medium. When light travels from a medium with a lower refractive index to a medium with a higher refractive index, it bends towards the normal. Conversely, when light travels from a medium with a higher refractive index to a medium with a lower refractive index, it bends away from the normal.

Inhomogeneous Refractive Index:
The refractive index of Earth's atmosphere is not constant and varies with altitude, temperature, and pressure. This variation in refractive index causes the air to act as a lens, focusing and bending the light that passes through it.

Oval Shape of the Evening Sun:
During sunset, when the Sun is close to the horizon, the light from the Sun has to pass through a larger portion of Earth's atmosphere compared to when it is directly overhead. This means that the light has to travel through air with varying refractive indices over a longer distance.

The inhomogeneous refractive index of the atmosphere causes different parts of the Sun's disk to be refracted by different amounts. The lower part of the Sun's disk, closer to the horizon, experiences a greater amount of refraction compared to the upper part. This differential refraction causes the lower part of the Sun to appear higher in the sky than the upper part, resulting in an oval or flattened shape.

Other Options:
- Red color of the evening Sun: The red color of the evening Sun is due to the scattering of shorter-wavelength blue and green light by Earth's atmosphere, leaving predominantly longer-wavelength red light to reach our eyes.
- Blue color of the sky: The blue color of the sky is primarily due to Rayleigh scattering, where shorter-wavelength blue light is scattered more than longer-wavelength red light by the molecules in Earth's atmosphere.
- Large apparent size of the evening Sun: The large apparent size of the evening Sun is an optical illusion known as the Moon illusion. It is not directly related to the inhomogeneous refractive index of Earth's atmosphere.