What is Liquefaction of Gases?

What is critical temperature ? What is its importance in liquefaction of gases?

Which of the following method is used for liquefaction of gases ?

Which of the following represent the suitable conditions for the liquefaction of gases ?

Critical Conditions and Liquefaction OF Gases,illustrations

Variation OF compressibility factor with temperature || Liquefaction OF gases & andrew's isotherms,critical temperature,pressure & volume

For three different gases values of van der Waals' constant 'a' and 'b' are given. What is the correct order of liquefaction of gases ? {:("Gases",a,b,),(X_(2),1.3,0.090,),(Y_(2),4.1,0.023,),(Z_(2),2.2,0.075,):}