The correct option is D R is always positive.
The curved surface formula is,
μ2v−μ1u=μ2−μ1R
where:
μ2: Refractive index of the medium in which the refraction takes place
μ1: Refractive index of the medium, from where the light is incident on the surface
R: Radius of curvature of the surface.
R can be positive or negative depending on the refracting surface. Assuming that the light is travelling from the left, if the refracting surface is concave, the value is negative and if it is convex the value if positive.