Which system in the aircraft is primarily responsible for navigation assistance?

The GPS system serves as the primary instrument for navigation assistance in aircraft. It provides pilots with precise location information, enabling them to determine their current position, track their course, and navigate effectively to their intended destinations. The system utilizes signals from satellites, allowing for accurate geolocation and routing.

In contrast, while the autopilot can assist in maintaining the aircraft's course and altitude based on navigational data, it is fundamentally reliant on the navigation systems, including the GPS, to receive positional information. Control surfaces are vital for maneuvering the aircraft but do not provide navigation assistance; they serve to change the aircraft's direction and attitude based on pilot input. Landing gear is essential for takeoff, landing, and ground operations, but it has no role in navigation. Thus, the GPS system is the correct answer as it directly facilitates navigation by offering real-time positional data.