Final answer:
A transistor is the device best described as an electrically operated switch, controlling the flow of current in a circuit by offering low resistance when closed and high when open.
Step-by-step explanation:
The device that can be best described as an electrically operated switch is a transistor. A transistor operates as an electric valve that controls the flow of current in a circuit, much like a mechanical valve controls the flow of water. When a switch, including a transistor, is closed, it offers very low resistance and allows electric current to flow through the circuit, powering the connected devices.
Conversely, when the switch is open, it presents high resistance, effectively stopping the flow of current and shutting off the devices in the circuit.