What occurs when the fuel switch is turned off?

When the fuel switch is turned off, the engine's operation is directly impacted as it essentially cuts off the fuel supply to the engine. This results in the disabling of the engine operation since the combustion process cannot continue without fuel. Therefore, the correct answer highlights that disabling engine operation is a necessary consequence of turning off the fuel switch.

The other choices may not accurately represent the function of the fuel switch. For example, activating an emergency fuel supply typically involves a separate switch or system designed to provide fuel in case of an emergency, rather than simply turning off the main fuel supply. Resetting the FADEC (Full Authority Digital Engine Control) is also unrelated directly to the fuel switch; it usually requires specific procedures or conditions to achieve. Lastly, clearing warning lights involves different systems and processes within the aircraft that don't occur automatically just by turning off the fuel switch.