How can you turn off a magneto in an aircraft?

The correct approach to turning off a magneto in an aircraft is by grounding the primary circuit. Magnetos are self-contained ignition devices used in aircraft engines, and they require a complete electrical circuit to function. When you ground the primary circuit of a magneto, you effectively create a short circuit that interrupts the ignition process. This action prevents the magneto from generating a spark, thus stopping engine operation effectively.

Other options are not appropriate for turning off a magneto. Cutting the fuel supply would stop fuel delivery to the engine but would not directly affect the magneto's ability to generate spark; the engine may still run momentarily until the remaining fuel is consumed. Disconnecting the battery is also ineffective in this context because magnetos operate independently of the aircraft's electrical system once they're running—hence, the battery's power would not influence magneto function directly. Redirecting the electrical current is an imprecise method and is not a standard practice for controlling magneto operation.