What is the primary difference between a relay and a solenoid?

The primary difference between a relay and a solenoid lies in their construction and function, particularly in the context of how they operate and respond to electrical inputs. Solenoids are electromechanical devices that consist of a coil of wire around a movable core or plunger, which moves when an electric current is passed through the coil. This movement converts electrical energy into mechanical energy, allowing the solenoid to perform actions such as pushing or pulling.

In contrast, relays are electrical switches that use an electromagnet to open or close circuits. While relays can indeed contain movable components, their primary function is to control the flow of electricity in a circuit rather than to physically move parts in the same way a solenoid does.

The other options distinguish characteristics that do not accurately portray the essential functions and differences between the two devices. For instance, relays can operate in both DC and AC circuits, and while relays can break circuits, solenoids typically do not function in this manner. Moreover, relays do not convert mechanical energy to electrical energy. Therefore, option C correctly identifies the feature of solenoids that sets them apart from relays, making it the right choice.