Dry ice is :

The correct answer is (a) Solid CO2.

Dry ice is the solid form of carbon dioxide. It is a white, odorless, and non-flammable solid that is used in a variety of applications, including refrigeration, cryotherapy, and theatrical effects. Dry ice is produced by compressing and cooling carbon dioxide gas until it liquefies. The liquid carbon dioxide is then allowed to expand and solidify into dry ice.

Solid ammonia (b) and solid sulfur dioxide (c) are not commonly referred to as dry ice. Dry CO2 gas (d) is not a solid.