Sun's halo is produced by the refraction of light

Answer: (c) Ice crystals in Cirrus clouds

Ice crystals in cirrus clouds refract light into a 22° halo.

The idea

A halo is an optical ring formed when light passes through suitably shaped ice crystals. Refraction bends the light through characteristic angles. High clouds are cold enough to contain these crystals. This differs from diffraction by small water droplets, which produces a corona.

High cloud contains ice crystals → Sunlight enters a crystal → Refraction changes its direction → Light exits along preferred angles → Many crystals produce a ring around the Sun

Why the others are wrong

(a) Stratus clouds are water droplets.

(b) Cirrocumulus can give coronas, but haloes come from cirrus/cirrostratus.

(d) Dust doesn’t make haloes.

Elimination technique

The refracting material must be ice crystals, not water vapour or dust, removing (a) and (d). Cirrus is the conventional high ice-cloud association used here; cirrostratus is a common actual halo-bearing cloud.

Also know

A common halo has an angular radius of about 22 degrees and is often seen through cirrostratus. A corona is usually a smaller coloured ring around the Sun or Moon caused by diffraction. A rainbow involves refraction and internal reflection in liquid raindrops and is seen opposite the Sun. Sundogs are bright patches produced by particular ice-crystal orientations.