Which device is used to control the flow of electricity in a circuit?

The device that controls the flow of electricity in a circuit is a switch. A switch functions by making or breaking the connection in an electrical circuit, allowing current to either flow through the circuit or be interrupted. This control means that a switch can turn devices on or off as needed, which is fundamental in electrical applications for safety, convenience, and functionality.

In contrast, a transformer is primarily used to change the voltage level in alternating current (AC) circuits, while a resistor is meant to limit the current flow and manage the resistance in a circuit. A capacitor is used for storing electrical energy temporarily and can influence circuit behavior in terms of energy storage and timing in AC circuits, but it does not serve as a direct control mechanism for electrical flow like a switch does. Therefore, the switch is key in determining whether electricity can flow in a circuit, making it the correct choice for this question.