The correct answer is option 4, which states that the gas used in a refrigerator is cooled down when expanded.

To understand this, let`s take a look at how a refrigerator works. A refrigerator operates on a principle called the refrigeration cycle. It involves the compression and expansion of a refrigerant gas to transfer heat from inside the refrigerator to the outside.

In the refrigeration cycle, the refrigerant gas is first compressed by a compressor, which increases its pressure and temperature. This compressed gas then flows through a series of coils, where it dissipates heat and cools down.

After the gas has been cooled down, it moves into an expansion valve. This valve reduces the pressure of the gas, causing it to expand rapidly. This expansion leads to a decrease in temperature, which is essential for cooling the refrigerator`s interior.

Therefore, it is the expansion of the gas that causes it to cool down in a refrigerator. This cooled gas then circulates through the refrigerator, absorbing heat from its surroundings and keeping the interior cool. Therefore, option 4 is the correct answer.