Explanation

(b) $Acceleration\ due\ to\ gravity\ g = \frac{GM}{R^{2}},\ \ \ \ \ \ M = \left( \frac{4}{3}\pi R^{3} \right)\rho$
$\therefore\ \ \ \ \ \ \ \ \ \ g = \frac{4G}{3}\frac{\pi R^{3}}{R^{2}}\rho$
$\Longrightarrow \ \ \ \ \ \ \ g = \left( \frac{4G\pi R}{3} \right)\rho\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ (\rho = average\ density)$
$\Longrightarrow g \propto \rho\ or\ \rho \propto g\ \ $