Explanation

(d) The atmosphere can be considered to consist of a number of parallel layers of air of different densities and therefore of different refractive indices. The density and the refractive index of layers decrease with altitude.
The rays of light coming from a star to the earth are thus continually refracted from the rarer to the denser layers and so they bend slightly towards the normal at each refraction from one layer to the next. Thus, they follow a curved path and reach the eyes of the observer at $O$ as shown in figure. Hence, the image of the star $S$ is seen as$\ S'$. But due to the wind and the convection currents in air the density of layers keep on changing and hence, the position of the stat $S'$ as seen, keeps on changing. These different images of the start give an impression to an observer that the star is twinkling.