What component holds back the movable plunger in a solenoid?

The component that holds back the movable plunger in a solenoid is the spring. In a solenoid, the spring plays a critical role by providing resistance to the plunger's movement, keeping it in its default position when the solenoid is not energized. When electrical current flows through the solenoid's coil, it creates a magnetic field that overcomes the force exerted by the spring. This allows the plunger to move, performing work such as opening or closing a mechanism.

While other components like magnets might be part of the solenoid itself, they do not play the specific role of holding back the plunger; instead, they contribute to the overall operation when energized. A plunger lock would imply a mechanism to secure the plunger in place, which is different from the function of a spring. Similarly, a gear mechanism typically refers to mechanical systems involving cogs and does not directly hold back a plunger in this context. Therefore, the spring's function as a return mechanism or a restraint is essential in a solenoid’s design, making it the correct answer.