Among the following, the gas expected to have the smallest value of the van der Waals constant $a$ is

For noble gases attraction arises only from dispersion forces, which increase with polarisability and hence with size. Neon is the smallest, so its $a$ ($0.21{\mathrm{L}}^2\mathrm{atm}\mathrm{mo}{\mathrm{l}}^{-2}$) is the least; xenon has the largest ($4.19$).