The correct answer is C. increase in speed and with decrease in radius of curve.
Superelevation is the banking of a road or railway track to counteract the centrifugal force experienced by vehicles traveling around a curve. The amount of superelevation required depends on the speed of the vehicle, the radius of the curve, and the coefficient of lateral friction between the tires and the road surface.
The coefficient of lateral friction is a measure of how slippery the road surface is. A higher coefficient of lateral friction means that the tires will have more grip on the road, and less superelevation will be required.
The speed of the vehicle is also a factor in determining the amount of superelevation required. The faster the vehicle is traveling, the greater the centrifugal force will be, and the more superelevation will be required to counteract it.
The radius of the curve is also a factor in determining the amount of superelevation required. The smaller the radius of the curve, the greater the centrifugal force will be, and the more superelevation will be required to counteract it.
Therefore, the value of required super-elevation increases with increase in speed and with decrease in radius of curve.