If Z is a compressibility factor ,van der waals equation at low pressure can be written as :

Question

$(a)\;Z=1+\large\frac{RT}{Pb} \qquad(b)\;Z=1-\large\frac{a}{VRT}\qquad(c)\;Z=1-\large\frac{Pb}{RT}\qquad(d)\;Z=1+\large\frac{Pb}{RT}$