The answer is B: 2R.
Consider a ring rolling without sliding.
Any particle of the ring is at the highest point of its path when it is diametrically opposite the point where the ring touches the surface. The distance between this particle and the point is 2R.
Since the ring rolls without sliding, the linear velocity of the point of contact between ring and surface is zero. This means this point serves as an instantaneous center of rotation, or the center of the curvature of the path. In other words, in any given moment, all particles of the ring are rotating around the point of contact between the ring and the surface.
Therefore, the radius of the curvature of the path of the point "on top" of ring is the distance between this point and the center of rotation, or 2R.