d
\( \text { Magnetic moment of coil } =\mathrm{NIA} \hat{\mathrm{j}}\)

\(=\mathrm{Nl}\left(\pi \mathrm{r}^{2}\right) \hat{\mathrm{j}} \)

Torque on loop (coil) \(=\overline{\mathrm{M}} \times \overline{\mathrm{B}}\)

\(=\mathrm{Nl}\left(\pi \mathrm{r}^{2}\right) \mathrm{B} \sin 90^{\circ}(-\hat{\mathrm{k}})\)

\(=\mathrm{NI} \pi \mathrm{r}^{2} \mathrm{B}(-\hat{\mathrm{k}})\)