What must always be done to ensure the circuit breakers are ready for use?

The correct answer is that all circuit breakers must be in the 'IN' position to ensure they are ready for use. This is crucial because circuit breakers are designed to protect electrical circuits by interrupting the flow of electricity in the event of an overload or fault. If a circuit breaker is in the 'IN' position, it indicates that the circuit is closed and power is being supplied, which is essential for proper operation.

Having all circuit breakers in the 'IN' position ensures that the systems they control are operational and can function as intended during flight. Additionally, when preparing for a flight, flight attendants and maintenance crews need to verify that all safety and operational systems are active, which relies on circuit breakers being engaged.

In contrast, if circuit breakers were visible but not in the correct position, or if they were pulled or marked in any specific way without being properly engaged, that could indicate a fault or disconnection that might hinder operations or safety. Thus, having them all positioned correctly is fundamental for the safety and functionality of the aircraft systems.