What type of device is designed to start and stop current flow?

A switch is a device specifically designed to start and stop the flow of electrical current within a circuit. It allows users to control the operation of electrical devices and systems by opening (turning off) or closing (turning on) the circuit. When the switch is in the "on" position, it completes the circuit, allowing electricity to flow; when in the "off" position, it breaks the circuit, stopping the current flow.

The other options serve different purposes. A power supply provides the necessary electrical energy to power a device, but it does not specifically control the flow of current like a switch does. A fuse is a safety device that protects circuits from overcurrent by melting and breaking the circuit when excessive current flows, but it is not used for normal operation to start and stop current flow. A circuit breaker also acts as a safety device which interrupts current in case of overload or short circuits, but it is typically more automated and intended to reset after an interruption, rather than being manually operated to start and stop current flow.