d
From figure
$\sin \frac{\theta}{2}=\frac{x}{r}$
$\Rightarrow x=\operatorname{rsin} \frac{\theta}{2}$
Hence new magnetic moment $M$
$=\mathrm{m}(2 \mathrm{x})=\mathrm{m} 2 \mathrm{rsin} \frac{\theta}{2}$
$=\mathrm{m} \cdot \frac{2 l}{\theta} \sin \frac{\theta}{2}=\frac{2 \mathrm{ml} \sin \theta / 2}{\theta}=\frac{2 \mathrm{M} \sin (\pi / 6)}{\pi / 3}=\frac{3 \mathrm{M}}{\pi}$
