Liquefaction Of Gases Liquid

If van der Waals constant "a" for NH₃, H₂ and CO₂ are respectively 4.17, 0.244, 1.36 and 3.59, then which one of the following gases is most easily liquefied?

$1.$ ${\mathrm{NH}}_3$
$2.$ ${\mathrm{H}}_2$
$3.$ ${\mathrm{O}}_2$
$4.$ ${\mathrm{CO}}_2$