The value of Vander waal's constant 'a' for  O2,N2,NH3 and CH4 are 1.360,1.390,4.17 and 2.253dm6 atmmol -2 respectively. The gas which can be most easily liquified is _____?

The Vander Waal's constant ‘a’ signifies the presence of attractive forces. Higher the value of ‘a’, higher will be the presence of intermolecular forces and so greater will be the ease of liquefaction.