The correct answer is: A. equally on inner and outer sides of the curve.
The extra widening is provided equally on inner and outer sides of the curve to provide a safe driving surface for vehicles. The widening helps to reduce the centrifugal force acting on the vehicles, which can cause them to skid or overturn. It also provides a wider margin of error for drivers, which can help to prevent accidents.
Option B is incorrect because the extra widening is not provided fully on the inner side of the curve. This would make the curve too narrow and could cause vehicles to skid or overturn.
Option C is incorrect because the extra widening is not provided fully on the outer side of the curve. This would make the curve too wide and could cause vehicles to lose control.
Option D is incorrect because the extra widening is not provided one-fourth on inner side and three-fourth on outer side of the curve. This would not provide a safe driving surface for vehicles.