Speed of sound is maximum in-

Sound is a mechanical longitudinal wave that requires a material medium to travel. Its velocity is directly proportional to the elasticity of the medium and inversely proportional to its density, represented by the relation:
$$v=\sqrt{\frac{E}{\rho }}$$

Because solids have a significantly higher elastic modulus ($E$) compared to liquids and gases, the speed of sound is maximum in solids. Sound cannot propagate through a vacuum ($v=0$).