In which of the following the transmission of sound is NOT possible ? Choose…

Concept: Sound is a mechanical wave: it travels only by making the particles of a material medium (solid, liquid, or gas) oscillate about their equilibrium positions and transfer that oscillation to their neighbours — through direct collisions in gases and liquids, and through elastic restoring forces between bonded particles in solids — passing the disturbance on as alternating regions of compression and rarefaction. Because this transfer mechanism always requires particles of a medium, sound cannot propagate through empty space.

Application: Checking each option against this requirement:

• Air is a gas made of freely moving molecules; their compressions and rarefactions carry sound at about 343 m/s at room temperature.

• Aluminium is a solid whose tightly packed, strongly bonded atoms transmit sound very efficiently via elastic restoring forces between neighbouring atoms, at around 6300 m/s — faster than in air or water.

• Water is a liquid whose molecules also support compressional waves; sound travels through water at about 1480 m/s, faster than in air.

• A vacuum is a region essentially devoid of matter. With no particles present to oscillate or collide, there is no mechanism left to carry a mechanical wave through it.

Cross-check: The classic demonstration of ringing a bell inside a sealed jar and pumping the air out confirms this — as the vacuum is created, the sound fades to silence even though the bell can still be seen striking, because no particles remain to carry the vibration outward. (This is unlike light, an electromagnetic wave, which needs no medium and travels through vacuum — for example, sunlight reaching Earth across space.)

Result: Among the options, a vacuum is the medium in which the transmission of sound is not possible.