Explanation

(b) $Acceleration\ due\ to\ gravity\ g = \frac{4}{3}\pi\rho GR$
$or\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ g \propto \rho$
$\therefore\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \frac{g_{1}}{g_{2}} = \frac{\rho_{1}}{\rho_{2}}$
$\frac{g_{1}}{g_{2}} = \frac{\rho}{2\rho}\ \ \ \ \ \ \ \ \ \ \left\lbrack \because\rho_{2} = 2\rho \right\rbrack$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ g_{2} = g_{1} \times 2 = 9.8 \times 2$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ g_{2} = 19.6\ m/s^{2}$