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The correct answer reflects the standard function and operation of automatic transfer switches (ATS). These devices are designed to automatically switch power supply from a primary source to a backup source, such as a generator, in the event of a power failure or disturbance.

Automatic transfer switches are typically electrically operated, meaning they can detect power interruptions and perform the switching action without manual intervention. This feature is essential for providing reliable power supply in critical situations such as hospitals or data centers, where an uninterrupted electric supply is crucial.

In addition, these switches are mechanically held, ensuring that the switch remains in position once it has been actuated, thereby reducing the risk of accidental switching due to transient power conditions. This mechanism provides stability during normal operation and ensures the system is resilient against momentary power fluctuations.

The other options do not align with the operational purpose or design of automatic transfer switches. For example, manual operation contradicts the 'automatic' aspect of the ATS, while electronically held suggests a reliance on electronic circuitry alone without mechanical assurance, which could compromise stability and reliability. Thus, the choice encompassing electrical operation with mechanical holding accurately encapsulates the intended design and functionality of automatic transfer switches.