Which type of wave requires a medium to travel?

Sound waves are mechanical waves that require a medium, such as air, water, or solid materials, to propagate. This is because sound waves transfer energy through the vibration of particles in the medium they travel through. In the case of sound, as the sound wave moves, it causes air molecules to compress and rarefy, enabling the transmission of the wave.

In contrast, electromagnetic waves, which include radio waves and light waves, do not need a medium to travel; they can propagate through a vacuum. This is due to their nature as fluctuations in electric and magnetic fields, which can exist without any material substance. Therefore, sound waves are unique in needing a medium for their transmission, differentiating them from all the other options listed.