The correct answer is: A. transient state of short-circuit current.
A circuit breaker is an electrical switch that is designed to protect an electrical circuit from damage caused by an overload or short circuit. Circuit breakers typically operate in the transient state of short-circuit current, which is the first few cycles after a short circuit occurs. During this time, the current can be many times the normal operating current of the circuit breaker. The circuit breaker must be able to interrupt this high current in order to protect the circuit.
Option B, sub-transient state of short-circuit current, is the period of time immediately after the transient state. During this time, the current is still high, but it is decreasing. The circuit breaker must also be able to interrupt the current during this time.
Option C, steady state of short-circuit current, is the period of time after the sub-transient state. During this time, the current has reached a steady state and is no longer increasing. The circuit breaker does not need to be able to interrupt the current during this time.
Option D, after D.C. component has ceased, is incorrect. The D.C. component of a short-circuit current is a small component that is present for a very short period of time. The circuit breaker does not need to be able to interrupt this component.