Which device allows control over when current flows in a circuit?

The option related to controlling when current flows in a circuit is the input device. Input devices, such as switches or sensors, serve as interfaces to provide control signals that manage the operation of a circuit. These devices enable the user or an automation system to initiate or interrupt the flow of electrical current, effectively acting as the means of control within the circuit.

In contrast, items like a control transformer are used to adjust voltage levels for power distribution and are not directly involved in the on-off functions of current flow. Output devices, such as actuators or motors, respond to control signals but do not directly manage when current is allowed to flow. Transformers are primarily used for stepping voltage up or down without directly controlling the current's on/off state. Hence, input devices play a crucial role in controlling the timing and flow of current in a circuit.