c
Binding energy of system \(=\frac{k e^2}{2 r}\) joule and \(\frac{k e^2}{2 r}=12.8\,ev\)

\(\frac{9 \times 10^9 \times\left(1.6 \times 10^{-19}\right)^2}{2 r}=12.8 \times 1.6 \times 10^{-19}\)

\(\Rightarrow r=\frac{9 \times 10^9 \times 1.6 \times 10^{-19}}{12.8 \times 2}\)

\(\Rightarrow r=\frac{9 \times 10^{-10}}{16}\)