A ring of mass $4 \mathrm{kg}$ is uniformly charged with a linear charge density $\mathrm{\lambda }=4\mathrm{ }\mathrm{C} {\mathrm{cm}}^{-1}$ and kept on a rough horizontal surface with a friction coefficient of $\mathrm{\mu }=\frac{\mathrm{\pi }}{4}$. A time-varying magnetic field $\mathrm{B}={\mathrm{B}}_0{\mathrm{t}}^2$ is applied in a circular region of radius $\mathrm{a}$ $\left(\mathrm{a}<\mathrm{r}\right)$ perpendicular to the plane of the ring as shown in the figure. Find out the time when the ring just starts to rotate on the surface. (take $\mathrm{a}=5 \mathrm{cm}$, $\mathrm{g}=10\mathrm{ }\mathrm{m} {\mathrm{s}}^{-2}$ and ${\mathrm{B}}_0=125$ SI units)

Hence, the correct answer is 4.