The stars twinkle in the night because:
Correct answer is (d)
In real Stars do not twinkle but as they are very far from the earth. So, they appear as point-sized sources of light and our atmosphere is largely heterogeneous having different densities and different refractive index. Due to these changing of physical properties, the amount of starlight reaching the earth changes from time to time and so stars sometimes appear brighter and at other times dull, which leads to the twinkling effect.