d
As we know for an ideal gas at constant
temperature, pressure $\mathrm{P}=\frac{\rho \mathrm{RT}}{\mathrm{M}}$
$\mathrm{P} \propto \rho$
$\therefore \mathrm{R}, \mathrm{T} \& \mathrm{M}$ are constant, so pressure $(P)$varies linearlywith density $(\rho)$.