If the EPR mode fails, the FADEC automatically reverts to the:

In the event that the EPR (Engine Pressure Ratio) mode fails, the FADEC (Full Authority Digital Engine Control) system is designed to revert to N1 mode. This is crucial for ensuring that the engine maintains an acceptable level of performance and safety.

N1 mode is primarily based on the rotational speed of the low-pressure compressor, which is a critical parameter for engine operation. It allows for monitoring and controlling the thrust produced by the engines, even if the EPR mode, which measures the ratio of the engine's pressure at the turbine outlet to its pressure at the compressor inlet, is unavailable.

This automatic transition ensures that the aircraft can still operate effectively and safely with the remaining data that the FADEC can access. The focus on maintaining flight safety and engine performance during a control failure highlights the advanced design and redundancy built into modern aircraft systems.

Other choices such as EPR mode or RPM mode are either the modes that the system is trying to replace or are not directly related to control for maintaining thrust in the case of failure.