The correct answer is option 3: refraction of starlight.

When starlight enters the Earth`s atmosphere, it encounters different layers of air with varying densities. These density differences cause the starlight to change direction or bend as it passes through the atmosphere. This phenomenon is called refraction.

Refraction of starlight is responsible for the twinkling effect we see when observing stars. As the starlight passes through the atmosphere and the different layers of air, it gets constantly refracted, resulting in the perceived twinkling or shimmering of the star.

Diffraction, mentioned in option 1, refers to the bending of waves when they encounter an obstacle or pass through an opening. It is not the primary cause of the twinkling of stars.

Reflection, mentioned in option 2, occurs when light bounces off a surface. While stars reflect light, this reflection does not cause the twinkling effect.

Dispersion, mentioned in option 4, is the separation of light into its component colors, as seen in a rainbow or when passing through a prism. Although dispersion can occur in the atmosphere, it does not directly cause the twinkling of stars.

Therefore, the correct answer is option 3: refraction of starlight, as it accurately explains the twinkle