What must be true for RTO to be selected?

For a RTO (Rejection of Takeoff) to be selected, it is essential that the aircraft is on the ground. This condition is crucial because RTO procedures are implemented during the takeoff phase of flight when the aircraft is still on the runway, in order to safely stop the aircraft before it becomes airborne.

When an aircraft is on the ground, the flight crew can assess a situation where it may be necessary to abort the takeoff, such as during a malfunction or unexpected event. Once the aircraft is airborne, the necessity for such an action changes significantly due to safety protocols and aircraft performance characteristics. Hence, being on the ground is a fundamental prerequisite for selecting RTO.

In the context of the other options, while brakes being applied may indeed be necessary for stopping the aircraft, it is not the primary condition that triggers the RTO. Similarly, without pedal input is more associated with control inputs during takeoff and does not pertain directly to the ability to select RTO. Therefore, the fact that the aircraft must be on the ground for RTO selection is pivotal to ensuring the safety and proper operation of the aircraft during the critical phases of takeoff.