Sound waves require what to travel through?

Sound waves are mechanical waves that require a medium to travel through, which can be solid, liquid, or gas. The molecules within these media vibrate and transfer the energy of the sound wave from one molecule to the next. This is why sound cannot propagate in a vacuum, where there are no molecules to transmit these vibrations.

While air is a common medium for sound, it is not the only one; sound can also travel through liquids like water and solids like metal. Therefore, sound waves need a general medium for propagation, making the option that states "a medium" the most accurate answer. The other choices do not encompass the essential characteristic that sound requires a physical medium for transmission.