Which component acts as a switch in a control circuit to turn motors on and off?

The component that acts as a switch in a control circuit to turn motors on and off is the contactor. A contactor is an electromechanical device designed specifically for controlling high-current loads, such as motors. When a control signal is applied, typically from a control circuit, the contactor closes its contacts, allowing current to flow to the motor and thereby turning it on. When the control signal is removed, the contacts open, stopping the current flow and turning the motor off. This functionality is essential in various industrial and commercial applications where motors need to be frequently started and stopped.

The other components listed do not serve the same purpose: a transformer is used to change voltage levels; a capacitor is generally employed for power factor correction or to smooth out voltage fluctuations; and a resistor is used to limit current flow in a circuit. None of these components can function as a switch to control the power supplied to a motor.