What is the function of the Ground Service Switch in the cockpit?

The Ground Service Switch in the cockpit serves the essential purpose of activating the ground power unit. This switch enables pilots and ground crew to efficiently manage the aircraft's electrical systems while it is on the ground, without needing to rely on the aircraft’s engines. By engaging the ground power unit, the switch facilitates the supply of power to onboard systems during pre-flight checks, maintenance, and boarding procedures, ensuring that critical systems remain operational without burning fuel or running the engines.

In contrast, the management of coolant systems typically falls under other controls and does not directly involve the Ground Service Switch. Similarly, controlling outside lighting and engaging de-icing procedures are functions managed separately through distinct controls. Therefore, the primary role of the Ground Service Switch is clearly identified as activating the ground power unit, making this option the most accurate in the context of cockpit functionality.