Which material will have maximum velocity of sound?

Sound is a mechanical wave, so it needs a medium, and its speed depends on the medium's elasticity and density. A stiffer material that passes vibrations quickly between neighbouring particles carries sound faster.

• Steel (D): a solid with speed of roughly 5,000 to 6,000 m/s. This is the maximum among the given options, so D is correct.
• Water (C): a liquid with speed of about 1,480 m/s at 20°C. It is less stiff than steel, so sound moves slower.
• Air (B): a gas with speed of about 343 m/s at 20°C. Gases have weak elastic restoring forces, so sound travels slowly.
• Vacuum (A): contains no particles, so no mechanical wave can propagate. The speed is zero.

The core idea is that in fluids v = √(B/ρ), where B is the bulk modulus and ρ the density, and in solids v = √(Y/ρ), where Y is Young's modulus. Solids usually win because their stiffness is very high. For Mains, this is why seismic P-waves move faster through the Earth's crust than through air, which is useful in questions on earthquakes and ocean acoustics.

Trap: many aspirants pick water because it seems denser and so must carry sound better. Density alone does not decide the answer; elasticity matters more, and solids win.