Ground spoilers can only extend when there is ________.

Ground spoilers are designed to assist in safely decelerating the aircraft during landing and provide increased aerodynamic braking. They only extend when weight is detected on the main landing gear to ensure that they only deploy when the aircraft is properly on the runway and stabilized. This prevents accidental deployment at inappropriate times during taxiing or takeoff, which could cause loss of control.

The activation mechanism relies on weight sensing to confirm that the aircraft is indeed on the ground. This design helps enhance safety by avoiding premature deployment during flight or in situations where the aircraft is still airborne. Therefore, the condition of having weight on the main gear is essential for the ground spoilers to function, making it the correct answer in the context of this question.