The ground crew interphone is activated ______.

The ground crew interphone is activated when the battery switch in either cockpit is switched on. This system relies on electrical power supplied by the aircraft's battery, allowing communication between the cockpit and the ground crew. The activation ensures that crew members can coordinate pre-flight procedures and any necessary communications before engine starts or while on the ground. This functionality is critical for maintaining clear and effective communication to enhance operational safety during ground operations.

In contrast, while the other choices mention conditions related to the flight deck security, engine start, or ground operations, they do not correctly describe the specific activation mechanism of the ground crew interphone system.