In the van der Waals equation for gases,what does the constant $b$ represent?

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

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:

The deviation will be maximum in the behaviour of a gas from the ideal gas equation $PV = nRT$?

Consider the following figure. Which of the following gases show negative as well as positive deviation from the ideal gas behaviour?

Under which conditions do intermolecular forces become effective?