What indicates that an aircraft is holding itself in the air?

The indication that an aircraft is holding itself in the air is when it is airborne. When an aircraft is airborne, it is actively flying and sustained above the ground by the lift generated by its wings. This is the state in which the aircraft is utilizing its engines and aerodynamic design to counteract gravity, allowing it to travel through the air.

In contrast, the other options describe states where the aircraft is not flying. When taxiing to the runway, the aircraft is moving on the ground and not yet in the air. If it is parked at the airport gate, it is stationary on the ground and not flying at all. Preparing for landing also indicates that the aircraft is transitioning back to the ground, not maintaining flight. Therefore, being airborne is the sole scenario in which the aircraft is sustaining itself in the air.