Moment of inertia of a solid sphere about a tangent is $I = (7/5)MR^2$. So, $I propto R^2$. The ratio of their densities must be $
ho_1/
ho_2 = 8/1$ for masses to be equal. The question is likely flawed and assumes equal density not equal mass. If masses are equal, $I_1/I_2 = R_1^2 / R_2^2 = R^2 / (2R)^2 = 1:4$. The provided answer key (b) suggests this interpretation.