Explanation

(d) Let radius of $_{4}^{9}{Be}$ nucleus be $r$. Then radius of germanium (Ge) nucleus will be$2r$.
Radius of nucleus is given by
$R = R_{0}A^{1/3}$
$\therefore\ \ \ \ \ \ \ \ \frac{R_{1}}{R_{2}} = \left( \frac{A_{1}}{A_{2}} \right)^{1/3}$
$\Rightarrow$ $\ \ \ \ \frac{r}{2r} = \left( \frac{9}{A^{2}} \right)^{1/3}\ \ \ \ \ \ \ \ \ \ (\because A_{1} = 9)$
$\Rightarrow \ \left( \frac{1}{2} \right)^{3} = \frac{9}{A^{2}}$
Hence, $A_{2} = 9\ \times \ (2)^{3}\ = 9 \times 8 = 72$
Thus, in germanium ($Ge$) nucleus number of nucleons is 72.