When should circuit breakers be closed and opened by the plane captain?

The correct choice focuses on the protocol for handling circuit breakers specifically before aircraft startup and shutdown. Closing circuit breakers is a crucial step to ensure that the aircraft systems are ready for operation as they control the electrical circuits necessary for the aircraft's functionality. Opening circuit breakers is equally important during shutdown to safely disconnect systems and prevent potential electrical issues when the aircraft is powered off.

This procedure is pivotal for ensuring the safety and efficiency of the aircraft's operations, as the components and systems involved must be appropriately managed to avoid malfunctions or hazards. While circuit breakers might be involved in routine inspections or maintenance checks, their handling during startup and shutdown periods is particularly critical for the operational integrity of the aircraft.