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 the most easily liquefied?

Van der Waals' constant 'a', signifies for intermolecular forces of attraction. Higher is the value of 'a', lower will be T, easier will be the liquefaction of gas. 

H₂, O₂, CO₂ gases are non-polar. NH₃ gas posses the dipole-dipole intermolecular forces of attraction. Hence, as compared to the other gases, NH₃ liquify most easily.