The correct answer is: A. 1 in 10.
The conical surface of the approach area rises outwards at a rate of 1 in 10. This means that for every 10 feet of horizontal distance, the surface rises 1 foot vertically. This slope is designed to provide a gradual and safe transition for aircraft as they land.
Option B: 1 in 15 is too shallow of a slope. This would not provide enough of a transition for aircraft, and could result in them landing too hard or too short.
Option C: 1 in 20 is also too shallow of a slope. For the same reasons as Option B, this would not provide enough of a transition for aircraft.
Option D: 1 in 25 is too steep of a slope. This could result in aircraft landing too hard or too long. Additionally, it would be more difficult for aircraft to taxi on a surface with such a steep slope.
Therefore, the correct answer is A. 1 in 10.