Which of the following formulas is used to calculate the compressibility factor?

Positive deviation from ideal behaviour is due to the fact that

Among the following,the maximum deviation from ideal gas behavior is expected from

The inversion temperature $T_i \, (K)$ of hydrogen is (given van der Waal's constants $a$ and $b$ are $0.244 \, atm \, L^2 \, mol^{-2}$ and $0.027 \, L \, mol^{-1}$ respectively).

For $1 \ mol$ of gas,the plot of $pV$ vs $p$ is shown below. $p$ is the pressure and $V$ is the volume of the gas. What is the value of compressibility factor at point $A$?

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 $'b'$?