The correct answer is: A. 1 in 15
Superelevation is the banking of a road to counteract the centrifugal force experienced by vehicles traveling around a curve. It is expressed as a ratio of the rise to the run, or the number of inches of rise per 100 feet of run.
The maximum limit of superelevation for mixed traffic in plain terrain is 1 in 15. This means that the road should be banked at a rate of 1 inch of rise for every 15 feet of run. This is the maximum limit because it is the point at which the centrifugal force experienced by vehicles is equal to the force of gravity. If the road is banked any more than this, vehicles will start to lose traction and could skid off the road.
Option B is incorrect because it is the minimum limit of superelevation for mixed traffic in plain terrain. Option C is incorrect because it is the maximum limit of superelevation for mountainous terrain. Option D is incorrect because camber is the cross-sectional slope of a road, and it is not the same as superelevation.