Identify an example of an actuator used with PLCs.

An actuator is a device that converts energy into motion to perform a specific action in a control system, such as those managed by a Programmable Logic Controller (PLC). In this context, electric motors, solenoids, and pneumatic cylinders serve as prime examples of actuators because they directly execute the physical action dictated by the PLC's control logic.

Electric motors can rotate or move components, solenoids can push or pull mechanisms to open or close valves, and pneumatic cylinders can produce linear motion with compressed air. Each of these devices is integral to the operation of automated systems, making them essential components for executing the commands given by a PLC.

Other choices, such as resistors and motor starters, do not function as actuators. Resistors are primarily used to limit electrical current in circuits rather than produce motion, while motor starters are devices designed to start electric motors but are not actuators themselves; they control the power supplied to the motor rather than performing the mechanical work directly. Variable frequency drives (VFDs) are used to control the speed of motors but again do not perform the physical movements; they modify the electrical input to actuators. This is why the correct choice is the one that directly aligns with the definition of an actuator.