The correct answer is option 2: constant change of refractive index of the medium between the stars and the Earth because of temperature variation.

Twinkling of stars, also known as stellar scintillation, is caused by the variation in the air density and refractive index along the Earth`s atmosphere. As the light from the stars passes through the Earth`s atmosphere, it encounters different layers of air with varying temperature and density. These variations cause the light to refract (bend) in different directions, leading to the twinkling effect.

The constant change in refractive index is primarily due to the temperature variation in the atmosphere. The temperature fluctuations cause the air to expand and contract, altering its density. This, in turn, causes the refractive index of the air to change continuously, leading to the twinkling of stars.

Options 1, 3, and 4 are not correct explanations for the twinkling of stars. Option 1 suggests that the fluctuation of surface temperature causes twinkling, which is not accurate as the source of twinkling lies in the Earth`s atmosphere. Option 3 suggests that the great distance of stars from the Earth causes twinkling, but distance alone does not cause twinkling; it is the atmospheric conditions that play a significant