The correct answer is D. electrons.
Cathode rays are a stream of negatively charged particles emitted from a cathode heated by an electric current. They were first observed in 1897 by J.J. Thomson, who showed that they were negatively charged particles much smaller than atoms. Thomson’s discovery of the electron was a major breakthrough in physics, and it led to the development of quantum mechanics.
Neutrons are uncharged particles that are found in the nucleus of an atom. Protons are positively charged particles that are also found in the nucleus of an atom. Positrons are positively charged particles that are the antiparticles of electrons.
Cathode rays are not made up of neutrons, protons, or positrons. They are made up of electrons.