a
(a) Suppose distance of closest approach is \(r\), and according to energy conservation applied for elementary charge.
Energy at the time of projection \(=\) Energy at the distance of closest approach
\(==>\) \(\frac{1}{2}m{v^2} = \frac{1}{{4\pi {\varepsilon _0}}}.\frac{{(Ze).e}}{r} \Rightarrow r = \frac{{Z{e^2}}}{{2\pi {\varepsilon _0}m{v^2}}}\)