Which of the following is a function of the navigation source during nav to nav transfer?

The function of the navigation source during nav to nav transfer is to revert to FMS 1 or 2. In the context of aviation systems, when a navigation source is switched from one system to another—such as from one Flight Management System (FMS) to another—this ensures that the aircraft has a reliable source of navigation data. The FMS plays a critical role in managing the flight plan, providing guidance, and ensuring situational awareness. By reverting to FMS 1 or 2, the aircraft ensures continuity in navigation and helps maintain operational safety during transitions between different navigation sources.

Maintaining altitude stability, locking the flight director, and activating profile descent are functions related to flight control but do not specifically pertain to the navigation source's role in this context. These actions involve different systems working together to ensure a safe and stable flight but are not directly executed as part of the navigation source transfer process. Thus, reverting to a reliable FMS is the essential function being highlighted in this situation.