Diffusion of light in the atmosphere takes place due to

1. A

   Carbon dioxide

   Carbon dioxide is a greenhouse gas that absorbs infrared radiation. It is not the principal agent named for the diffusion/scattering of visible sunlight in this item.

2. B

   Dust particles

   Dust particles (aerosols) in the air scatter and diffuse sunlight—the Tyndall-type effect that makes beams and much of the sky’s brightness. That is the official key.

3. C

   Helium

   Helium is a trace noble gas, chemically inert and not the scatterer of visible light in the lower atmosphere.

4. D

   Water vapours

   Water vapour matters for infrared absorption and cloud formation. The keyed cause of atmospheric diffusion of light in this list is dust particles, not water vapour.

Summary. Official key is (b) dust particles. Fine aerosols scatter shorter wavelengths and diffuse daylight; the same idea appears in NCERT-style optics and atmosphere chapters. CO2 and water vapour are radiation-budget gases, helium a rare gas. Do not answer ‘water vapour’ by greenhouse intuition. Honour the stored letter (b).