How is the FADEC powered when the N2 is below 50%?

The correct choice is that the FADEC (Full Authority Digital Engine Control) is powered by the aircraft's main batteries electrical system when the N2 (the speed of the high-pressure compressor) is below 50%.

This is important because the FADEC system requires electrical power to operate its various functions, including managing fuel flow, ignition, and throttle settings. At low N2 values, the engines may not produce enough power or electrical output to sustain the FADEC function, so the system relies on the aircraft's battery to ensure continuous operation. This allows the FADEC to maintain control of the engine during critical phases of operation, such as engine start or in the event of a power loss from the engines.

Other options like using an external power source or a backup generator are generally applicable in specific situations or aircraft configurations but do not represent the standard operational procedure for FADEC power at low N2 speeds.