Due to which of the following, sky appears to be blue?

The correct answer is (d), scattering.

Scattering is the phenomenon of redirection of a beam of light (or other radiation) by particles of a much smaller wavelength. Sunlight is scattered by the tiny molecules of air in Earth’s atmosphere. Blue light is scattered more than the other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.

Refraction is the bending of light as it passes from one medium to another. Reflection is the bouncing of light off a surface. Diffraction is the bending of light as it passes through a narrow opening or around an obstacle.

I hope this helps!