Which set of conditions represents the easiest way to liquefy a gas?
A.low temperature and high pressure
B.high temperature and low pressure
C.low temperature and low pressure
D.high temperature and high pressure

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Hint: Liquefaction process is defined as the process used for generating liquid either from solid or gas. Here we are going to discuss the liquefaction process converting gas into liquid.

Complete step by step answer:
Before directly talking about the answer, let us discuss Linde's process.
Linde’s process is used for the liquefaction of gas. The gas is compressed, cooled and expanded which results in the reduction of temperature. The molecules occupy less space because they move slowly as the temperature is decreased. Therefore, gas gets converted into liquid form.
As we increase the pressure, the intermolecular distance is decreased. On the other hand, as we decrease the temperature, there will be a decrease in the energy of gaseous molecules which decreases their intermolecular distance and therefore gas liquifies.
Hence, the correct option is (A), that is, low temperature and high pressure.

Additional information: For analyzing the fundamental properties of a gas, liquefaction process is used. For example, oxygen is present in the form of gas but if we increase the pressure and decrease the temperature, the oxygen gas will get converted into its liquid form.
To liquify gas, there is one more process which is known as Claude’s process in which the gas expands isentropically in the chamber. To liquify the air, it is expanded at supercritical pressures. Final liquefaction takes place by isenthalpic expansion.

Critical temperature is a temperature at which a gas gets converted into liquid
For liquefaction of gas, it requires cooling as well as compression. Gas which shows positive deviation from compressibility factor $(z)$ will show both cooling and compression.