The van der Waals' constant $\prime \mathrm{a}\prime$is directly proportional to the intermolecular force of attraction; hence, the gas with greater value of $\prime \mathrm{a}\prime$ can be most easily liquefied.The values of Vander Waal's constant $\text{'}\mathrm{a}\text{'}$ for the given gases are:
    Gases                    Vander Waal's Constant $\text{'}\mathrm{a}\text{'}$
A. $\mathrm{Ar}$                           $1.35 {\mathrm{L}}^2$ atm ${\mathrm{mol}}^2$
B. ${\mathrm{CH}}_4$                       $2.25{\mathrm{L}}^2$ atm  ${\mathrm{mol}}^2$
C. ${\mathrm{H}}_2\mathrm{O}$                       $5.536{\mathrm{L}}^2$ atm ${\mathrm{mol}}^2$
D. ${\mathrm{C}}_6{\mathrm{H}}_6$                       $18.57{\mathrm{L}}^2$ atm ${\mathrm{mol}}^2$
$\therefore$Increasing order of $\text{'}\mathrm{a}\text{'}$ for the given gases
$\mathrm{A}<\mathrm{B}<\mathrm{C}<\mathrm{D}$