Under what condition must a circuit breaker not be reset?

The correct choice is based on the need for safety and adherence to established procedures when dealing with electrical systems on the aircraft. Circuit breakers are critical components that protect the electrical system from overloads and faults. If a circuit breaker has tripped, it indicates a potential issue that must be diagnosed and resolved before it is reset.

Resetting a circuit breaker without following explicit procedures can lead to further complications, including electrical fires, component failures, or other hazardous situations. Therefore, it is essential to ensure that any action taken regarding circuit breakers aligns with the specific guidelines provided in the aircraft's operating manual or maintenance procedures. These procedures are designed to ensure the safety of the aircraft, the crew, and passengers.

In contrast, the other conditions related to the other choices may still allow for a circuit breaker to be reset if proper checks and balances are maintained, which is why they do not represent the same level of absolute caution as the correct choice.