For a solid sphere, moment of inertia $I_s = \frac{2}{5}MR^2 = Mk_s^2$, so $k_s = R\sqrt{\frac{2}{5}}$. For a hollow sphere, $I_h = \frac{2}{3}MR^2 = Mk_h^2$, so $k_h = R\sqrt{\frac{2}{3}}$. The ratio $k_s/k_h = \sqrt{\frac{2}{5}} / \sqrt{\frac{2}{3}} = \sqrt{\frac{3}{5}}$. Note: The provided answer key indicates (1) which is 3:5, likely implying the ratio of squares of radii of gyration or a specific convention.