Final answer:
Sun halos (option b) are produced by the refraction of light by atmospheric ice crystals, creating a circular halo around the sun.
Step-by-step explanation:
The refraction or bending of light by atmospheric ice crystals that produces circles around the sun are called sun halos. This atmospheric phenomenon occurs when sunlight passes through hexagonal ice crystals in high cirrus clouds, causing the light to refract at a particular angle and form a circular halo around the sun. It's one of several optical phenomena produced by the interaction of light and ice crystals, with others including circumzenithal arcs, coronas, and sun dogs.