c
(c) After redistribution new charges on spheres are \(Q{'_1} = \left( {\frac{{10}}{{10 + 20}}} \right) \times 10 = \frac{{10}}{3}\,\mu C\)
and \(Q{'_2} = \left( {\frac{{20}}{{10 + 20}}} \right) \times 10 = \frac{{20}}{3}\,\mu C\)
Ratio of charge densities \(\frac{{{\sigma _1}}}{{{\sigma _2}}} = \frac{{Q'_1}}{{Q'_2}} \times \frac{{{r_2}^2}}{{{r_1}^2}}\)
\( = \frac{{10/3}}{{20/3}} \times {\left( {\frac{{20}}{{10}}} \right)^2} = \frac{2}{1}\)\(\left\{ {\sigma = \frac{Q}{{4\pi {r^2}}}} \right\}\)