In Van der Waal's equation of state of the gas law, the constant 'b' is a measure of
Volume occupied by the molecules
Look at the options carefully, option b and c are definitely talking about the constant 'a'. We are left with two options.
This is direct theory if you recall the derivation of the mean free path - we consider the excluded volume to calculate mean free path right?
'b' is a representation of that volume itself.