The minimum radius ( R in metres) of the valley curve for cubic parabola is given by Where,
L = total length of valley curve
N = deviation angle in radians or tangent of the deviation angle

Correct option is B

​$$\text{For a cubic parabola used in valley curve design (especially sag curves under headlight sight distance or comfort criteria),} \\\text{the radius of curvature at the lowest point (where curvature is maximum) is given by the formula:} \\R = \frac{L}{2N} \\\text{Where:} \\R = \text{Radius at the lowest point (minimum radius)} \\L = \text{Total length of valley curve} \\N = \text{Deviation angle (in radians or } \tan\theta\text{, i.e., algebraic difference of slopes)}$$​​