What is the probable cause if the engine continues to run after the magneto switch is turned to the "OFF" position?

When the magneto switch is turned to the "OFF" position, it is designed to interrupt the electrical connection to the magneto, effectively stopping the spark that ignites the fuel in the engine. If the engine continues to run after this switch is turned off, it indicates that there is still a functioning path for the spark to reach the engine.

A broken magneto ground wire is the most probable cause in this scenario. The ground wire plays a crucial role in completing the electrical circuit that grounds the magneto when the switch is turned off. If this wire is broken or disconnected, the magneto may not be effectively grounded, allowing the engine to continue igniting fuel and running despite the switch being in the OFF position.

The other potential causes in the options do not directly explain the scenario where the engine continues to operate as expected when the magneto switch is turned off. A fuel leak would typically lead to an engine failure or sputtering, while an over-voltage condition in the battery is unlikely to directly influence the ignition process in the context of magneto operation. The malfunction of spark plugs could lead to engine performance issues but would not result in continued operation when the ignition system is disabled.