Bany ideal gas, whether monoatomic, diatomic or polyatomic
b
(b)
Mayer's relation,
$C_p-C_V=R$ is true only for ideal gases whether they are monoatomic, diatomic or polyatomic as for real gases.
$C_p-C_V=T V \beta^2 k$
where, $k=$ isothermal compressibility and $\beta=$ isobaric thermal expansion coefficient.
So, correct option is $(b)$.