ক্যাথোড রশ্মি হলো -

Electrons.

Cathode rays are a stream of electrons emitted from the cathode of a vacuum tube when a high voltage is applied across the electrodes. The electrons are accelerated towards the anode, and can be focused to form a beam. Cathode rays were discovered in 1876 by William Crookes, and were used in the development of the cathode-ray tube (CRT), which was used in televisions and oscilloscopes.

Electromagnetic waves are a type of radiation that travels in a vacuum at the speed of light. Positrons are antiparticles of electrons, and protons are positively charged particles that are found in the nucleus of an atom.

Therefore, the answer is Electrons.