The discovery of the proton is historically linked to the study of canal rays (also called positive rays or anode rays), first observed by the German physicist Eugen Goldstein in 1886, using a discharge tube fitted with a perforated cathode.

How canal rays form: In a discharge tube, fast-moving electrons (cathode rays) travel from the cathode toward the anode and collide with neutral gas atoms along the way, knocking electrons off them. This creates positively charged ions, which — being oppositely charged to the electrons — are accelerated in the opposite direction, i.e., toward the cathode. Since the cathode used by Goldstein had small holes or "canals" drilled into it, these positive ions streamed through these holes and continued moving beyond the cathode, forming a faint glow that could be studied — hence the name canal rays or anode rays.

Key experimental finding: Unlike cathode rays (whose charge-to-mass ratio was always the same, regardless of the gas used), the charge-to-mass ratio of canal ray particles varied depending on the gas filling the tube — because these are actual ionized gas atoms, not a single universal particle. However, when the tube was filled with hydrogen gas (the lightest element), the resulting canal ray particles showed the largest possible charge-to-mass ratio among all gases tested. This particle — the ionized hydrogen atom, carrying a single unit of positive charge — was identified as a new fundamental particle and later named the proton (by Rutherford, building on this work).

Thus, the discovery of the proton is correctly associated with canal rays carrying positively charged particles.

The other options are incorrect: cathode rays carry negatively charged particles (electrons), not protons, ruling out any option that pairs cathode rays with proton discovery; canal rays consist of charged ions, not neutral particles, so they cannot be associated with the (electrically neutral) neutron, which in any case was discovered much later through entirely different experiments involving beryllium bombardment; and cathode rays are streams of electrons, not photons, so the claim that they contain photons emitted from metal electrodes is also false.