Rainbow is caused due to:
A) Reflection of sunlight by air molecules.
B) Dispersion of sunlight from water drops.
C) Refraction of sunlight from water drops.
D) Refraction of sun rays by air molecules.

Hint: In this question, we will use the concept of the refraction to obtain the correct answer. First discuss the refraction phenomenon of light and then discuss how a ray enters the water droplets. After that, discuss how the light rays dispersed from the water droplets and the seven different wavelengths rays come out and form a rainbow.

Complete solution:
As we know that Rainbow is caused due to the dispersion of sunlight from water drops. Some parts of the sunlight fall on water drops and then enter the water drops. Now the colour components of the sunlight are dispersed and forming a rainbow.

So when the sunlight falls on water drop it enters the water drop due to the refraction phenomenon. So here the water drop acts as a prism. So when the white light passes through this prism, the white light is dispersed into its different component colours-orange, red, yellow, green, blue, violet. This is known as dispersion phenomenon. This phenomenon is caused by the change of speed of light ray of different wavelengths.
So we can say a rainbow is formed due to the dispersion of sunlight from water droplets and refraction of sunlight into the water droplets.

Hence, Option (B) is the correct option.

Note: As we know that when the light enters the water droplets it comes from air medium to much denser medium water, so this is the reason why the light enters the water droplets.