Which is an electromagnetic coil that contains a movable plunger?

• A. Servo
• B. Tachometer
• C. Solenoid
• D. Transformer

Answer: (C)

This question tests understanding of devices that use an electromagnetic coil to create linear motion. An electromagnetic coil with a movable plunger is a solenoid. When current flows, the magnetic field pulls the plunger into the coil, producing linear actuation that can open or close valves, operate switches, or drive other mechanisms. A servo is a motor with feedback control used to position a load precisely, not a simple coil-and-plunger actuator. A tachometer is a speed sensor, not an actuator. A transformer uses coupled windings on a magnetic core to transfer energy without moving parts. Therefore, the device described is a solenoid.