Stars appear to twinkle in the night sky due to:

The correct option is C Refraction
Twinkling of stars is due to atmospheric refraction. There are different layers in the atmosphere with varying temperature and density. This results in the variation of refractive index of the atmosphere with time. So, when the light from a star passes through these different layers, it experiences different levels of refraction. Thus, the image of the star gets distorted constantly in brightness and position which makes the star look like its twinkling.