Cathode rays were first observed in the late 19th century using low-pressure gas discharge tubes, where a high voltage is applied between two electrodes sealed inside a glass tube containing gas at very low pressure. Experiments showed that a glowing stream originated from the cathode (the negative electrode) and traveled toward the anode (the positive electrode), causing the glass wall opposite the cathode to fluoresce.

The definitive identification of cathode rays as streams of electrons was established by J.J. Thomson in 1897, through a landmark set of experiments. Thomson demonstrated several crucial properties:

• Cathode rays travel in straight lines in the absence of external fields, but are deflected by both electric and magnetic fields in a manner consistent with negatively charged particles — confirming they carry negative charge and are not neutral radiation like light.
• The rays are produced regardless of the material used for the cathode or the type of gas inside the tube, showing that the particles making up cathode rays are a universal constituent of matter, common to all atoms — this was revolutionary, as it implied atoms were not indivisible, contrary to earlier belief.
• Thomson measured the charge-to-mass ratio (e/m) of these particles and found it to be constant and about a thousand times larger than that of the lightest known ion (hydrogen), implying the particle was either highly charged or extremely light — leading to the conclusion that these were tiny, negatively charged particles far lighter than atoms, later named electrons.

Since cathode rays travel from the cathode toward the anode, and consist of negatively charged, low-mass particles, the correct description is that they are streams of electrons moving from cathode toward anode.

The incorrect options can be ruled out as follows: cathode rays are not photons, since photons are uncharged and travel at the speed of light without being deflected by electric or magnetic fields, unlike the observed cathode ray behaviour; they are not protons, because protons are far heavier, positively charged, and are instead responsible for the separately observed canal rays (moving from anode toward cathode); and they are not neutrons, since neutrons carry no charge and would not respond to electric/magnetic deflection at all, nor were they even discovered until decades later (1932, by Chadwick).