# A square loop of side $2a$, and carrying current $I$, is kept in $XZ$ plane with its centre at origin. A long wire carrying the same current $I$ is placed parallel to the z-axis and passing through the point $(0,b,0)$,$(b> > a).$ The magnitude of the torque on the loop about z-axis is given by : Option: 1   Option: 2 Option: 3 Option: 4

$\begin{array}{l} \vec{\tau}=\overrightarrow{\mathrm{M}} \times \overrightarrow{\mathrm{B}} \\ \\ =4 \mathrm{a}^{2} \mathrm{I} \times \frac{\mu_{0} \mathrm{I}}{2 \pi \mathrm{b}} \end{array}$

$= \frac{2\mu _{0}I^{2}a^{2}}{\pi b}$

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