The correct answer is A. Dry ice is solid carbon dioxide. It is a white, odorless, and non-flammable solid that is used as a refrigerant and in dry ice blasting. Dry ice sublimates at -78.5°C, which means that it changes directly from a solid to a gas without going through a liquid phase. This makes it ideal for use in applications where it is important to keep things cold without getting them wet.
Option B, crystalline NaCl, is table salt. It is a white, crystalline solid that is used as a food additive and in many industrial processes. Option C, ice frozen at -50°C, is simply ice that has been frozen to a very low temperature. Option D, solid H2O, is water ice. It is a colorless, odorless, and tasteless solid that is found in nature in the form of glaciers, snow, and ice cubes.