The correct option is A H2O>CO2>Ar
Van der Waals' constant 'a' is a measure of intermolecular forces of attractions. Stronger is the force of attraction, greater will be ‘a’ and easily a gas can be liquified. In H2O, hydrogen bonding exists, hence H2O possesses strongest intermolecular forces of attraction. In CO2, due to resonance, it is easily polarizable and hence stronger intermolecular forces of attraction. Ar has only van der Waals' forces of attraction. So, the forces of attraction for H2O are higher, followed by CO2 and then Ar.
Hence, the values of a are in the order: H2O>CO2>Ar.