Answer: The correct answer is vacuum.
Explanation:
Sound wave is a longitudinal wave in which the particles vibrate parallel to the direction of the motion of the wave. Sound wave consists of compression and rarefaction.
It needs a medium to travel. It can travel in solid, liquid and gas. It cannot travel in vacuum.
Denser the medium, more will be the speed of the wave.
Solid is more denser in comparison to liquid.
The speed of the sound in the solid medium is more in comparison to the liquid. The speed of the sound in liquid is more than the speed in the gas.
Therefore, vacuum is least effective medium for traveling sound waves.
