Electromagnetic waves are waves that are capable of propagating through a vacuum or through different media. These waves are made up of changing electric and magnetic fields that are perpendicular to each other and to the direction of the wave's propagation. Sound waves, water waves, and tidal waves are all mechanical waves, meaning they require a medium to propagate through. X-rays, on the other hand, are electromagnetic waves, which means they do not require a medium to propagate through and can travel through a vacuum. Therefore, the correct answer is x-rays.