Hi aviator3233,
I guess this is question came from an FAA ATP test.
The correct answer is "on ground". But just reviewing the concept of speedbrake, remember that the speedbrake is a function of the flight spoilers, which are also used for rolling combined with the ailerons; the spoilers used on ground are called ground spoilers, they are spoilers that only open on ground. Usually the flight spoilers are also used as "ground spoilers" while on ground, but at an higher angle than in flight. On the 737 aircraft, for instance, the speed brake lever have a flight detent and you should not extend the speedbrakes beyond this detent in flight, because beyond this detent the flight spoilers will open in an higher angle that is designed to be used only on ground. Once you land this kind of airplane, you should see this lever moving automatically to the full extended position; in this occasion, the flight spoilers will be being used as ground spoilers as I mentioned above.
So as you can see, this question is a little bit tricky. But for FAA purposes, choose the "on ground" answer.