Which of the following waves requires a medium to travel?

Sound waves are classified as mechanical waves, which means they require a medium (such as air, water, or solids) to transmit energy. This is due to the fact that sound waves propagate through the vibration of particles within the medium. When a sound is produced, it generates pressure changes that move through the medium, allowing the sound to be heard.

In contrast, both light waves and X-rays are types of electromagnetic waves that do not require a medium to travel. These waves can propagate through the vacuum of space because they are oscillations of electric and magnetic fields. Radio waves also belong to the category of electromagnetic waves and behave similarly to light and X-rays; they can travel through a vacuum and do not need a medium.

The requirement of a medium for sound waves is evidenced by the fact that sound cannot travel in a vacuum, such as in outer space. Thus, sound waves are unique among the options as they need a material medium for their propagation.