In the van der Waals equation,which of the following will have the highest value of '$a$'?

Which of the following represents the Van der Waals equation for $n$ moles of a gas?

Write the van der Waals equation.

Pressure versus volume graph for a real gas and an ideal gas are shown in the figure. Answer the following questions on the basis of this graph:
$(i)$ Interpret the behaviour of real gas with respect to ideal gas at low pressure.
$(ii)$ Interpret the behaviour of real gas with respect to ideal gas at high pressure.
$(iii)$ Mark the pressure and volume by drawing a line at the point where real gas behaves as an ideal gas.

For real gases,the relation between $P$,$V$ and $T$ is given by the van der Waals equation,$(P + \frac{an^2}{V^2})(V - nb) = nRT$. For the following gases $CH_4, CO_2, O_2, H_2$,which gas will have $(i)$ highest value of $'a'$ $(ii)$ lowest value of $'b'$?

At relatively high pressure,the van der Waals equation reduces to