Which refrigerant is highly flammable and is often subject to careful handling?

The correct choice indicates that hydrocarbons (HCs) are highly flammable and require careful handling. Hydrocarbons, such as propane (R-290) and isobutane (R-600a), are used as refrigerants due to their low environmental impact and efficiency. However, their flammable nature means that precautions must be taken during storage, handling, and installation to prevent fire hazards.

The other refrigerants mentioned do not share this same level of flammability. For instance, R-717, or ammonia, is not classified as flammable, although it can pose toxic risks. R-123 is a hydrochlorofluorocarbon (HCFC) with a low flammability risk, and R-500 is a blend that includes non-flammable components. Therefore, it is essential to recognize that the need for careful handling is specifically relevant to hydrocarbons due to their flammability.