b
Energy stored in magnetic field \(=\frac{1}{2} \mathrm{Li}^{2}\)

Energy stored in electric field \(=\frac{1}{2} \frac{q^{2}}{C}\)

\(\therefore \quad \frac{1}{2} L i^{2}=\frac{1}{2} \frac{q^{2}}{C}\)

Also \(q=q_{0} \cos \omega t\) and \(\omega=\frac{1}{\sqrt{L C}}\)

On solving \(t=\frac{\pi}{4} \sqrt{L C}\)