The Vander Waal's constants for four gases P, Q, R and S are 4.17, 3.59, 6.71 and 3.8  atm L2mol−2  Therefore, the ascending order of their liquefaction is

Easily liquefiabie gases have greater intermolecular forces which is represented by high value of 'a'. The greater the value of 'a' more will be liquefiability. so, the order is Q