Which type of circuit breaker trips immediately when needed under all circumstances?

The choice indicating that none of the circuit breaker types trip immediately under all circumstances is linked to how each circuit breaker operates.

Magnetic circuit breakers are designed to trip quickly in response to short circuits or overloads because they use an electromagnet that activates and breaks the circuit when a certain level of current is exceeded. However, they may not trip as quickly in situations where the overload is gradual, as they are more sensitive to sudden surges in current.

Thermal circuit breakers, on the other hand, rely on heat generated by the current flowing through them. They trip based on the heating of a bimetallic strip, which takes time to heat up sufficiently to trigger the trip mechanism. This inherent delay means they are not as immediate in their response compared to magnetic breakers in certain scenarios.

Thus, while magnetic breakers are designed for fast trips, they are not guaranteed to trip instantly in all situations, particularly in gradual overload cases. Similarly, thermal breakers have built-in delays due to their operational principles. Therefore, the most accurate response is that neither type of circuit breaker can universally guarantee an immediate trip under all circumstances, resulting in the conclusion that none of the options correctly describes a circuit breaker that meets that criteria.