b
\(\overrightarrow{\mathrm{T}}=\overrightarrow{\mathrm{M}} \times \overrightarrow{\mathrm{B}}=-\mathrm{MB} \sin \theta\)

\(\mathrm{I} \alpha=-\mathrm{MB} \sin \theta\)

for small \(\theta\)

\(\alpha=-\frac{\mathrm{MB}}{\mathrm{I}} \theta\)

\(\omega=\sqrt{\frac{\mathrm{MB}}{\mathrm{I}}}=\sqrt{\frac{(\mathrm{I})\left(\pi \mathrm{R}^{2}\right) \mathrm{B}}{\left(\frac{\mathrm{mR}^{2}}{2}\right)}}\)

\(\omega=\sqrt{\frac{2 \mathrm{I} \pi \mathrm{B}}{\mathrm{m}}}\)

\(\therefore \mathrm{T}=\frac{2 \pi}{\omega}=\sqrt{\frac{2 \pi \mathrm{m}}{\mathrm{IB}}}\)