The phenomenon responsible for the blue colour of the sky

Scattering of light

The correct answer is Option (3) → Scattering of light

The blue colour of the sky is due to the scattering of sunlight by molecules and small particles in the atmosphere.

According to Rayleigh's law, the intensity of scattered light is inversely proportional to the fourth power of the wavelength:

$I \propto \frac{1}{\lambda^4}$

Since blue light has a shorter wavelength than red, it is scattered more strongly, making the sky appear blue.