(3) P-V diagram of the gas is a straight line passing through origin.

Hence P $\propto$ V or PV^–1 = constant

Molar heat capacity in the process PV^x = constant is

$C=\frac{R}{\gamma -1}+\frac{R}{1-x}$Here $\gamma =1.4$ (For diatomic gas)

$\Rightarrow C=\frac{R}{1.4-1}+\frac{R}{1+1}\Rightarrow C=3R$