The twinkling of a star is due to which of the following phenomenon?

Atmospheric refraction

The twinkling of a star is due to atmospheric refraction of starlight.

The starlight, on entering the earth’s atmosphere, undergoes refraction continuously before it reaches the earth. The atmospheric refraction occurs in a medium of gradually changing refractive index. Since the atmosphere bends starlight towards the normal, the apparent position of the star is slightly different from its actual position.