c
Let \(q_{1}\) and \(q_{2}\) be charge on two spheres of radius \('r'\) and \('R'\) respectively As, \(q_{1}+q_{2}=Q\)

and \(\sigma_{1}=\sigma_{2} \quad\) [Surface charge density are cqual \(]\)

\(\therefore \frac{q_{1}}{r \pi r^{2}}=\frac{q_{2}}{4 \pi R^{2}}\)

So, \(q_{1}=\frac{Q r^{2}}{R^{2}+r^{2}}\) and \(q_{2}=\frac{Q R^{2}}{R^{2}+r^{2}}\)

Now, potential, \(V=\frac{1}{4 \pi \varepsilon_{0}}\left[\frac{q_{1}}{r}+\frac{q_{2}}{R}\right]\)

\(=\frac{1}{4 \pi \varepsilon_{0}}\left[\frac{Q r}{R^{2}+r^{2}}+\frac{Q R}{R^{2}+r^{2}}\right]\)

\(=\frac{Q(R+r)}{R^{2}+r^{2}} \frac{1}{4 \pi \varepsilon_{0}}\)