A real gas can be liquefied:
A.Under adiabatic expansion
B.Above critical temperature
C.When cooled below critical temperature under apply pressure
D.At temperature lower than critical temperature at pressure higher than critical temperature

Hint:At first think about real gas and its formation and properties. Real gases are non- ideal gases whose molecules occupy space and have interactions, consequently and they do not obey the ideal gas law.

 Complete step by step answer:
The behavior of real gas can be explained by the interactions between the gaseous molecules.
Liquefaction of gases is the process by which a gas is converted into a liquid. Liquefaction of a gas occurs when its molecules are pushed closed together. A real gas can be liquefied when cooled below critical temperature under application of pressure.
So the correct answer is option D.

Additional Information:- It is important to note that a gas must be considered as a real gas when it is approaching its condensation point. Almost all gases must be considered as real gases as they approach their critical points. In other situations in which gases can be considered as real gases include situations in which the pressure applied to the gas is very high, in order to explain the Joule Thomson effect. It is also important to note that the deviation of a real gas from the behavior of an ideal gas can be expressed in the terms of the compressibility factor.
The graph between pressure and volume is,


Note:
Don’t get confused with the terms real gas and ideal gas. A real gas is a gas which does not obey the law of ideal gas. Ideal gas equation is $PV = nRT$ where P is the pressure, V is volume, n is amount of substance, R is ideal gas constant and T is temperature.