Given van der Waals constant for $\mathrm{N}{\mathrm{H}}_3,{\mathrm{H}}_2,{\mathrm{O}}_2$ and $\mathrm{C}{\mathrm{O}}_2$ are respectively 4.17, 0.244, 1.36 and 3.59, which one of the following gases is most easily liquefied?

Here $\mathrm{N}{\mathrm{H}}_3$ is most easily liquefied as for it the value of 'a' is maximum.
van der waal constant ‘a’, signifies intermolecular forces of attraction.
Higher is the value of ‘a’, easier will be the liquefaction of gas.
Liquefaction $\propto$ ‘a’-value
$\mathrm{N}{\mathrm{H}}_3>\mathrm{C}{\mathrm{O}}_2>{\mathrm{O}}_2>{\mathrm{H}}_2$