(c) $g = \frac{GM}{R^{2}}$; If $R$ decreases then $g$ increases. Taking logarithm of both the sides;
$\log g = \log{G + \log{M = - 2\log R}}$
Differentiating it we get; $\frac{dg}{g} = 0 + 0 - \frac{2dR}{R}$
$= - 2\left( \frac{- 2}{100} \right) = \frac{4}{100}$
$\therefore\%$ increase in $g = \frac{dg}{g} \times 100 = \frac{4}{100} \times 100 = 4\%$