Which waves does not belong to the category of the other three

Which one of the following waves does not belong to the category of the other three?

  1. X-rays
  2. Microwaves
  3. Radio waves
  4. Sound waves

Answer

Sound waves do not belong in the electromagnetic spectrum. The full electromagnetic spectrum contains all of the various wavelengths and frequencies of electromagnetic radiation. 

The Sound wave is a mechanical wave. It requires a medium to travel while electromagnetic waves can travel in a vacuum.

The correct option is D.