What should be disconnected from the A/C prior to operation of either ATM?

Disconnection of the external power source from the aircraft is essential prior to operating either ATM (Auxiliary Turbine Module). This is primarily due to safety and operational considerations. The external power unit supplies electricity to various systems within the aircraft, and connecting it while operating the ATM could lead to electrical interference or an unintentional back-feed of power. This can jeopardize the integrity and performance of the APU, potentially leading to equipment malfunctions or safety hazards during operation.

Additionally, disconnecting the external power ensures that the APU can operate independently and that the onboard electrical systems are managed by the APU's own power generation capabilities. It allows for a clear and safe transition of power management to the APU, which is critical for ensuring stable and reliable operation.

The other options, while important for different aspects of operation, do not pose the same direct threat as the external power source.