b
\(T = 2\pi \sqrt {\frac{I}{{pE}}} \) \(t = \frac{T}{4} = \frac{\pi }{2}\sqrt {\frac{I}{{pE}}} \)\(I = M\,{\left( {\frac{L}{2}} \right)^2} + M\,{\left( {\frac{L}{2}} \right)^2} = \frac{{M{L^2}}}{2}\) , \(p = qL\) \(==>\) \(t = \frac{\pi }{2}\sqrt {\frac{{M{L^2}}}{{2 \times qL \times E}}} = \frac{\pi }{2}\sqrt {\frac{{ML}}{{2qE}}} \)