Explanation

(b) Thermal capacity = Mass $\times$ Specific heat
Due to same material both spheres will have same specific heat. Also mass = $Volume\ (V) \times Density(\rho)$
$\therefore$ Ratio of thermal capacity
$= \frac{m_{1}}{m_{2}} = \frac{V_{1}\rho}{V_{2}\rho} = \frac{\frac{4}{3}\pi r_{1}^{3}}{\frac{4}{3}\pi r_{2}^{3}} = \left( \frac{r_{1}}{r_{2}} \right)^{3} = \left( \frac{1}{2} \right)^{3} = 1\ :8$