What is a characteristic of all refrigerants used in cooling systems?

The correct choice highlights a fundamental property of refrigerants in cooling systems: they change state during heat transfer. Refrigerants are unique fluids that can transition between liquid and gas phases during their refrigeration cycle. This phase change is essential for their ability to absorb and release heat efficiently.

In cooling systems, refrigerants absorb heat from the environment (often from the space being cooled) when they evaporate from a liquid to a gas. This process allows them to capture the heat, which is then released when the refrigerant condenses back into a liquid in the condenser. This cycle of absorption and heat release is what makes refrigeration possible, and it underlies the operation of various cooling systems, such as air conditioners and refrigerators.

The other characteristics presented do not universally apply to all refrigerants. While some refrigerants may have particular boiling points or specific behaviors at various temperatures, the essential and defining characteristic for refrigerants in the context of heat transfer across the system is their ability to change state. This property is what allows them to effectively move heat from one location to another, enabling the cooling process.