Which of the following is not an EM wave?

The correct answer is Option (4) → Cathode rays

An electromagnetic wave is a wave of oscillating electric and magnetic fields that travels in vacuum at the speed of light and carries no charge. X-rays, gamma rays and microwaves all belong to the electromagnetic spectrum, differing only in wavelength.

Cathode rays are different: they are a stream of fast-moving electrons, which are charged particles with mass. They travel slower than light and are deflected by electric and magnetic fields, as J.J. Thomson used to measure e/m, whereas electromagnetic waves are not deflected by such fields.

Note: The same question was also asked in CUET (UG) 2023 on 05-06-2023 (Slot 3).

NCERT (2026-27):
Introduction – “… These cathode rays were discovered, in 1870, by William Crookes who later, in 1879, suggested that these rays consisted of streams of fast moving negatively charged particles. The British physicist J. J. Thomson (1856-1940) confirmed this hypothesis.”