In a cathode ray tube, the number of electrons that reach the fluorescent screen is controlled by the:

Grid

Explanation:

The number of electrons attaining the fluorescent light is regulated by the grid in a cathode ray tube. When the heater is supplied with current, it allows the electrons to emit from the cathode. Before approaching the anode the electrons pass through grid opening. The number of electrons attempting to reach the anode may also be regulated by managing the number of electrons passing through the grid.