A (n) is a substance that moves heat by changing state from a liquid to gas when absorbing heat and from gas to liquid when rejecting heat.

The correct choice is a refrigerant. A refrigerant plays a crucial role in heat transfer systems, particularly in refrigeration and air conditioning. It has the unique property of changing states between liquid and gas as it absorbs and releases heat. When it absorbs heat, it evaporates from a liquid to a gas, effectively removing heat from the environment (like in a refrigerator). Conversely, when it releases heat, it condenses from a gas back into a liquid, displacing heat back into the surroundings. This cycle of absorbing and rejecting heat is fundamental to the operation of cooling systems.

The other choices do not fit this description accurately. A condenser is a component that condenses refrigerant vapor into liquid but does not itself move heat by changing state. A superheater is used to increase the temperature of gas without changing its state, while expansion material generally refers to components that might serve to accommodate thermal expansion but do not inherently possess the heat transfer capabilities associated with changing states like a refrigerant does.