Given van der Waals' constant for NH3,H2,O2 and CO2 are respectively 4.17,0.244,1.36 and 3.59, which one of the following gases is most easily liquefied?

van der Waals' constant 'a' signifies the intermolecular forces of attraction between the particle of gas. So, higher the value of 'a', easier will be the liquefaction of gas.