The speed of sound is maximum in

Sound is a mechanical wave — it travels by making the particles of a medium vibrate and pass that vibration to neighbouring particles. How fast it travels depends on the medium's elastic modulus (its resistance to being compressed) and its density, roughly following v = √(E/ρ): a medium with tightly bound particles and strong restoring forces carries the vibration faster than one with loosely bound, sparse particles.

Among the four media given, steel has its atoms locked into a rigid, tightly packed lattice held by strong metallic bonds. That gives it a very high elastic modulus, so a disturbance passes from atom to atom very rapidly — making the speed of sound through steel the highest of the options offered.

Comparing every option by its own value:

• Water (liquid): sound travels at about 1480 m/s — molecules can still flow past each other, so the coupling is weaker than in a rigid lattice.

• Air (gas): sound travels at about 343 m/s at room temperature — molecules are far apart with only occasional collisions, giving the weakest coupling among the material media.

• Vacuum: there are no particles to vibrate at all, so sound cannot propagate through it — its speed is undefined, not maximum.

Ordering the four media by speed of sound gives steel > water > air > vacuum (no propagation), so the speed of sound is maximum in steel.