Under which conditions does a gas show the greatest deviation from the ideal gas equation $PV = nRT$?

The correction factor '$a$' in the van der Waals equation for real gases corresponds to:

For $1$ mol of a real gas kept at constant temperature $T$,the graph of $PV$ vs $P$ intersects the $PV$ axis at $20$. Hence,the temperature $T$ is ($R$ = universal gas constant).

Four gases $P, Q, R$ and $S$ have almost same values of $b$ but their $a$ values ($a, b$ are van der Waals' constants) are in the order $Q < R < S < P$. At a particular temperature among the four gases, the most liquefiable one is

Which among the following statements is/are incorrect regarding real gases?
$(i)$ Their compressibility factor is never equal to unity $(Z \neq 1)$.
$(ii)$ The deviations from ideal behavior are less at low pressures and high temperatures.
$(iii)$ Intermolecular forces among gas molecules are equal to zero.
$(iv)$ They obey Van der Waals equation,$PV = nRT$.

Under which conditions does the $van \ der \ Waals$ equation reduce to the ideal gas equation?