Which GPS button is primarily used during holding and nonprecision approaches?

The OBS button, or "Observe" mode, is specifically designed to assist pilots in holding patterns and during nonprecision approaches. Activating OBS mode allows the pilot to select and track a specific waypoint or approaching course without the GPS automatically sequencing to the next waypoint. This gives pilots more control over their navigation, which is crucial in maintaining the correct path during holding patterns and following specific approach procedures.

In holding patterns, pilots need to precisely manage their route and timing, and using OBS mode facilitates that by allowing manual navigation adjustments as necessary. During nonprecision approaches, where the precision of the guidance is less than that of precision approaches, OBS mode allows the pilot to focus on the defined approach path more effectively.

The other options, while related to navigation, do not provide the same level of manual control necessary for holding and nonprecision approaches. The other buttons enable different functionalities, such as programming routes or directing GPS navigation in a more automated fashion, which may not be suitable in the nuanced operations required during holding or nonprecision approaches.