Which of the following waves is NOT a mechanical wave?

An electromagnetic wave is indeed not a mechanical wave. Mechanical waves, such as sound waves, water waves, and seismic waves, require a medium (like air, water, or solid earth) to travel through. These waves cause oscillations in the medium as they propagate, meaning they cannot exist in a vacuum.

In contrast, electromagnetic waves, which include visible light, radio waves, and X-rays, do not need a medium; they can travel through the vacuum of space. This fundamental characteristic distinguishes electromagnetic waves from mechanical waves, as they are formed by oscillating electric and magnetic fields rather than by the vibration of particles in a medium. Thus, electromagnetic waves are capable of spreading energy through empty space, making them uniquely different from the other types of waves listed.