Which of the given sets of temperature and pressure will cause a gas to exhibit the greatest deviation from ideal gas behavior?

The deviation from the ideal gas equation $PV = nRT$ is maximum under which of the following conditions?

The graph between $P$ and $V$ below the critical temperature is:

The relation between pressure exerted by an ideal gas $(P_{ideal})$ and observed pressure $(P_{real})$ is given by the equation
$P_{ideal} = P_{real} + \frac{an^2}{V^2}$
$(i)$ If pressure is taken in $N \ m^{-2}$,number of moles in $mol$,and volume in $m^3$,calculate the unit of $a$.
$(ii)$ What will be the unit of $a$ when pressure is in atmosphere and volume in $dm^3$?

Under which conditions do intermolecular forces become effective?

What is the unit of the Van der Waals constant $a$?