The correct answer is option 2: atmospheric refraction of starlight.
When starlight enters the Earth`s atmosphere, it encounters different layers of air with varying densities. This variation in density causes the path of the starlight to bend or deviate slightly. This phenomenon is known as atmospheric refraction.
As the starlight passes through the atmosphere, the bending of the light causes the apparent position of the star to shift slightly. This continuous change in position creates the perception of twinkling. The light appears to flicker or twinkle as it reaches our eyes.
It is important to note that twinkling is more pronounced for stars that are closer to the horizon. This is because the starlight has to pass through a larger portion of the Earth`s atmosphere, experiencing more refraction and causing greater fluctuations in position.
Therefore, option 2 correctly explains that the twinkling of a star is due to the atmospheric refraction of starlight.