Explanation

(c) Mass = Volume $\times$ Density $\Rightarrow M = \frac{4}{3}\pi r^{3} \times \rho$
As the density remains constant
$\therefore M \propto r^{3}$
$\therefore\frac{r_{1}}{r_{2}} = \left( \frac{M_{1}}{M_{2}} \right)^{1/3} = \left( \frac{M}{8M} \right)^{1/3} = \frac{1}{2}$ …(i)
Terminal velocity, $v_{T} = \frac{2}{9}\frac{r^{2}(\rho - \sigma)g}{\eta}$
Where, $r =$ radius of a spherical body
$\rho =$ density of the material of body
$\sigma =$ coefficient of viscosity of the medium
As $\rho,\sigma,\eta$ remain constant
$\therefore v_{T} \propto r^{2},\ \therefore\frac{v_{T_{1}}}{v_{T_{2}}} = \left( \frac{r_{1}}{r_{2}} \right)^{2}$
$\frac{v}{nv} = \left( \frac{r_{1}}{r_{2}} \right)^{2}$ or $\frac{1}{n} = \left( \frac{1}{2} \right)^{2}$ [Using (i)]
Or $n = 4$