What is the value of $Z$ for an ideal gas and a real gas?

The given graph represents the variation of $Z$ (compressibility factor = $\frac{PV}{nRT}$) versus $P$,for three real gases $A, B$ and $C$. Identify the only incorrect statement.

For real gases,the van der Waals equation is written as $\left( p + \frac{a n^2}{V^2} \right) (V - nb) = nRT$,where $a$ and $b$ are van der Waals constants. Two sets of gases are:
$(I)$ $O_2, CO_2, H_2, He$
$(II)$ $CH_4, O_2, H_2$
The gases given in set-$I$ are arranged in increasing order of $b$,and the gases given in set-$II$ are arranged in decreasing order of $a$. Select the correct order from the following:

At Boyle's temperature,the compressibility factor $Z$ for a real gas is:

The following data is obtained for one mole of a gas. The gas deviates from ideal behavior in the pressure (in bar) range:

$P \text{ (bar)}$	$\frac{PV}{RT}$
$1$	$1$
$2$	$1$
$3$	$1$
$3.1$	$1.2$
$3.5$	$1.4$
$4.0$	$1.5$

The Van der Waals equation reduces itself to the ideal gas equation at