Option 2 (Half of its radius of curvature) -> For both concave and convex spherical mirrors, the focal length (f) is always half the radius of curvature (R), expressed as f = R/2. This relationship is derived from paraxial ray approximation where rays close to the principal axis reflect through or appear to diverge from the focal point, which lies midway between the pole and center of curvature of the mirror -> correct