Which device can automatically control electrical operation without manual input?

The device that can automatically control electrical operation without manual input is a relay. A relay functions as an electromagnetic switch that can open or close circuits in response to an electrical signal. This means that when a certain voltage or current is detected, the relay will trigger a mechanism to either connect or disconnect an electrical circuit automatically, enabling control over various electrical systems without the need for human intervention.

Relays are widely used in control systems and automation processes because of their ability to handle signaling and control tasks efficiently. For example, they can be employed in lighting control, motor control, or in alarm systems, where precise and automatic responses are necessary for safety and functionality.

In contrast, the other devices mentioned do not possess the same level of automatic control capability. A circuit breaker is designed primarily for protection against overcurrents but requires manual reset after tripping. A transformer is used to change voltage levels in electrical systems and does not control operations by itself. A capacitor stores electrical energy and can smooth out voltage fluctuations but does not provide automatic control functionality. Hence, the choice of a relay as the correct answer is well-founded, given its role in automation and control.