In a Cathode Ray Tube (CRT), the electron beam emitted by the heated cathode diverges naturally and must be squeezed to a fine spot before it strikes the phosphor screen; otherwise the trace would be a large blur. This is done by an electrostatic focusing system that behaves like an optical lens for electrons.

The focusing arrangement uses three cylindrical anodes in line along the beam axis:

1. A first (pre-)accelerating anode held at a high positive potential.
2. A focusing anode in the middle held at a lower, adjustable potential.
3. A second accelerating anode again held at a high positive potential.

Because the middle electrode sits at a lower voltage between two high-voltage electrodes, the equipotential surfaces in the gaps curve, creating regions of non-uniform field. These curved fields act on the electrons like a converging electrostatic lens, first bending the diverging electrons inward and then straightening them so they converge to a sharp point on the screen. Varying the middle anode's voltage changes the lens strength, which is exactly what the FOCUS control on the CRO front panel does.

This matches the correct choice: a focusing anode at lower potential placed between two high-potential accelerating anodes.

Using only high-potential accelerating anodes (or two equal-potential anodes with no focusing electrode) provides acceleration but no lens action, so the beam cannot be brought to a fine focus. Placing the fluorescent screen at a lower potential after the anodes describes the screen/aquadag arrangement, not the focusing mechanism, and does not converge the beam.