c
$P-V$ diagram is a straight line passing through the origin. Hence, $P \propto V$
$\Rightarrow P V^{-1}=$ constant.
Molar heat capacity for $P V^{x}=$ constant is
$C=\frac{R}{\gamma-1}+\frac{R}{1-x}$
$=\frac{R}{1.4-1}+\frac{R}{1+1}$
$=\frac{R}{0.4}+\frac{R}{2}$
$=\frac{10 R}{4}+\frac{R}{2}$
$=\frac{5 R}{2}+\frac{R}{2}=3 R$