The correct answer is: A. 10 to 400 pF.
Air capacitors are generally available in the range of 10 to 400 pF. They are made up of two metal plates separated by a thin layer of air. The capacitance of an air capacitor is proportional to the area of the plates and inversely proportional to the distance between them. Air capacitors are relatively inexpensive and have a high capacitance-to-volume ratio. However, they are also relatively lossy and have a low breakdown voltage.
Option B is incorrect because air capacitors are not generally available in the range of 1 to 20 pF. Option C is incorrect because air capacitors are not generally available in the range of 100 to 900 pF. Option D is incorrect because air capacitors are not generally available in the range of 20 to 100 pF.