The rated operating duty cycle of a circuit breaker refers to the sequence of operations that the circuit breaker undergoes during its normal operation. The correct sequence of operations in this case is option 'A', which is O = t CO-T-CO.

Explanation:
- The first operation is 'O', which stands for open. This means that the circuit breaker is in the open position, interrupting the flow of current.
- After a certain time period, denoted by 't', the circuit breaker undergoes the next operation 'CO', which stands for close-open. This means that the circuit breaker closes momentarily and then opens again.
- Following the CO operation, the circuit breaker goes through the 'T' operation, which stands for time delay. The circuit breaker remains closed for a specific time period, in this case, 3 minutes.
- After the time delay, the circuit breaker undergoes another 'CO' operation, closing momentarily and then opening again.
- Finally, the circuit breaker goes through the 'CO' operation one more time, closing momentarily and then opening.

This sequence of operations represents the rated operating duty cycle of the circuit breaker. The circuit breaker opens initially, undergoes a close-open operation, remains closed for a specific time period, and then goes through two more close-open operations before returning to the open position. This sequence ensures the proper functioning and protection of the circuit breaker during normal operation.

It is important to note that the time delay, denoted by 'T', is typically longer than the momentary close-open operations, denoted by 'CO'. This allows the circuit breaker to provide a short delay before opening again, allowing for the stabilization of the electrical system before interrupting the flow of current.

In summary, the correct sequence of operations for the rated operating duty cycle of a circuit breaker is O = t CO-T-CO. This sequence ensures the proper functioning and protection of the circuit breaker during normal operation.