Explanation

(c) After redistribution, charges on them will be different, but they will acquire common potential
$i.e.\ k\frac{Q_{1}}{r_{1}} = k\frac{Q_{2}}{r_{2}} \Rightarrow \frac{Q_{1}}{Q_{2}} = \frac{r_{1}}{r_{2}}$
As $\sigma = \frac{Q}{4\pi r^{2}} \Rightarrow \frac{\sigma_{1}}{\sigma_{2}} = \frac{Q_{1}}{Q_{2}} \times \frac{r_{2}^{2}}{r_{1}^{2}} \Rightarrow \frac{\sigma_{1}}{\sigma_{2}} = \frac{r_{2}}{r_{1}} \Rightarrow \sigma \propto \frac{1}{r}$
$i.e.$ surface charge density on smaller sphere will be more