What defines a circuit breaker in terms of operation?

A circuit breaker is designed to automatically open and close a circuit in response to changing conditions, such as overcurrent or fault conditions. This functionality is crucial for protecting electrical systems from damage due to overloads or short circuits.

In addition to automatic operation, circuit breakers can also be operated manually. This allows operators to intentionally open or close a circuit when necessary, such as for maintenance or testing purposes. Therefore, the ability of a circuit breaker to function in both automatic and manual capacities is what makes option C the correct choice.

Other options are limited in scope. For example, stating that a circuit breaker can only open a circuit automatically would neglect its ability to close circuits and its manual operation. Describing it as operated manually only ignores its critical automatic functions. Lastly, claiming that it is used solely for system testing does not account for its primary role in protection and control of electrical circuits during normal operation.