Explanation

(d) Terminal velocity, $v_{T} \propto r^{2}$
$\ or\ \ \ \frac{v_{T_{1}}}{\ v_{T_{2}}} = \frac{r_{1}^{2}}{r_{2}^{2}}$
$\therefore\ \ \ \ \sqrt{\frac{9}{4} = \frac{r_{1}}{r_{20}}}$
$or\ \ \ \ \ \ \ \ \frac{r_{1}}{r_{2}} = \frac{3}{2}$
$\therefore\ \ \ \ \ \ v = \frac{4}{3}\pi r^{3}$
$or\ \ \ \ \ \ \ \ \frac{v_{1}}{v_{2}} = \frac{r_{1}^{3}}{r_{2}^{3}} = \frac{27}{8}$