Explanation

(a) Radius of nucleus $R\ = \ R_{0}A^{1/3}$
Where $R_{0} = \ 1.2\ \times \ 10^{- 15}\ m$
Volume of nucleus $(V)$ = $\frac{4}{3}\pi R^{3}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \ \frac{4}{3}\pi\left\lbrack \ R_{0}A^{1/3} \right\rbrack^{3}$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ = \frac{4}{4}\pi R_{0}^{3}A$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ V \propto A$