Which of the following is not an EM wave?

The correct option is Cathode rays.

To understand why, let us analyze the physical nature of each option:
1. X-rays, Gamma rays, and Microwaves: These are all forms of electromagnetic (EM) radiation. They consist of synchronized oscillations of electric and magnetic fields that propagate through a vacuum at the speed of light. They do not carry mass.
2. Cathode rays: These are streams of electrons emitted from the negative electrode (cathode) in a vacuum tube. Since electrons are particles that possess both mass and a negative electric charge, cathode rays represent a flow of particles (particulate radiation) rather than electromagnetic waves.

Therefore, cathode rays are not electromagnetic waves.