The cathode ray tube is vacuum tube that contains a stream of electrons. These electrons are negatively charged particles. In order to direct these electrons to a specific pathway, a magnetic field is being used to exert forces over the negatively charged particles that are in motion.
Therefore, the answer is C. MAGNETIC FIELD
J.J. Thompson discovered the electron, which a constituent of the subatomic particles under the cathode ray tube experiment.
Therefore, the answer is C. STUDY OF MOVING ELECTRONS IN THE PRESENCE OF MAGNETIC FIELD.