The van der Waals parameters for gases W, X, Y and Z are

Gas	$a(atm L^2 mo{l}^{-2})$	$b(L mo{l}^{-1})$
W	4.0	0.027
X	8.0	0.030
Y	6.0	0.032
Z	12.0	0.027

Which one of these gases has the highest critical temperature?

Critical temperature of a gas is the temperature at or above which vapor of the gas cannot be liquefied, no matter how much pressure is applied. $Tc=\frac{8a}{27Rb}$, a and b are specific constants for each gas.