Circuit breakers are held in the closed position by ______.

The correct choice indicates that circuit breakers are held in the closed position by mechanical props. This is a fundamental aspect of circuit breaker design, where the mechanical props or mechanisms work to maintain the structure of the breaker in a closed state until a specified condition occurs, such as an overload or a fault in the circuit. This ensures that the circuit remains closed and operational under normal conditions.

Mechanical props can include various components such as levers or linkage systems that provide the necessary force to keep the contacts together. The utility of mechanical props enhances the reliability and effectiveness of the circuit breaker, ensuring that the device can handle the mechanical stresses associated with high currents.

In contrast, while magnetic attraction and a maintaining solenoid could hypothetically be involved in certain types of circuit breakers, they are not the primary means of holding traditional circuit breakers closed. Springs can play a role in the mechanism but are typically used more for the opening mechanism or assisting in the snapping action. Thus, the emphasis on mechanical props underscores their critical role in ensuring the reliability and functionality of electrical systems in which circuit breakers operate.