What condition activates the anti-skid system during landing?

The correct condition that activates the anti-skid system during landing is when a wheel spin-up signal is present. The anti-skid system is designed to prevent the wheels from locking up and to maintain maximum braking efficiency without losing traction. When the aircraft lands and the wheels begin to contact the runway, the anti-skid system monitors the rotation of the wheels. If a wheel spin-up signal is detected, it indicates that the wheel is starting to spin after touchdown, and the anti-skid system can engage to help manage braking effectively.

This function is crucial because it helps the pilot maintain control of the aircraft even under wet or slippery runway conditions. The system can modulate the brake pressure dynamically to optimize stopping distance while ensuring safety, making this condition vitally important during the landing phase of flight.