Which of the following conditions must be met for RTO (Rejected Takeoff) to be armed?

For RTO (Rejected Takeoff) to be armed, it is essential that the thrust is at idle or in reverse. When the thrust levers are in this position, the aircraft's systems are prepared to execute the rejected takeoff procedure, which enables the pilots to safely abort the takeoff if needed. This condition ensures that the engines are not contributing additional thrust that could complicate stopping the aircraft rapidly.

In the context of the other options, while they may involve elements of the takeoff phase or aircraft state, they do not specifically position the aircraft for executing a rejected takeoff. For example, being airborne (WOW in the air) would make RTO irrelevant since the procedure applies during ground operations. A speed above 60 knots is significant in terms of safety and performance, but alone it does not activate the RTO system. Similarly, pressing the brakes is a part of the stopping process but does not initially arm the RTO unless the thrust levers are appropriately set. Thus, idle or reverse thrust is the critical factor for RTO arming.