Right option is (b) False
The best I can explain: Speed of longitudinal waves is v = √(B/ρ), where B is bulk modulus.
For an isothermal process PV=const.
On differentiating, we get: PdV + VdP = 0.
∴ P = -dP/(dV/V) = B.
∴ v = √(B/ρ) = √(P/ρ).
We have used the ideal gas equation, so this is only valid for an ideal gas.