Gas used in a refrigerator is cooled down when expanded.

Explanation:

Gas is the working substance in a refrigerator that undergoes a thermodynamic cycle to remove heat from the inside of the refrigerator and expel it to the surroundings. This cycle consists of four main processes: compression, condensation, expansion, and evaporation.

1. Compression:
The gas is initially compressed by a compressor, which increases its pressure and temperature. This process is responsible for raising the temperature of the gas.

2. Condensation:
The high-pressure gas then enters the condenser, where it is cooled down by releasing heat to the surrounding environment. This cooling causes the gas to condense into a liquid.

3. Expansion:
The condensed liquid then passes through an expansion valve or throttle valve, which reduces its pressure and temperature. As the liquid expands and flows through the valve, it undergoes a phase change and transforms back into a gas.

4. Evaporation:
The low-pressure gas now enters the evaporator, which is located inside the refrigerator. As the gas absorbs heat from the refrigerator's interior, it undergoes evaporation, cooling down the surroundings.

The key point to note here is that during the expansion process, the gas experiences a decrease in pressure and temperature. This decrease in temperature is what causes the gas to cool down. The gas absorbs heat from the refrigerator's interior, maintaining a low temperature inside the refrigerator.

Therefore, option 'D' is the correct answer as it correctly states that the gas used in a refrigerator cools down when expanded.