When the axis of rotation of the loop is along B by $30^{\circ}$ about an axis perpendicular to its plane make no change in the angle made by axis of the loop with the direction of magnetic field. So, the work done to rotate the loop is zero.
$$
\mathrm{W}=\mathrm{MB} \cos 90^{\circ}=0
$$
When rotation not along B.
The work done to rotate the loop in uniform magnetic field $\mathrm{W}=\mathrm{MB}\left(\cos \theta_1-\cos \theta_2\right)$, where signs are as usual. So the workdone is
$$
\begin{aligned}
\mathrm{W} &=\mathrm{MB} \cos \theta\left(\theta=30^{\circ} \text { given }\right) \\
&=\mathrm{MB} \cos 30^{\circ}=\frac{\mathrm{MB} \sqrt{3}}{2}
\end{aligned}
$$