Magnetic dipole moment

Net magnetic moment $= 2M\cos\frac{\theta}{2}$, where $\theta$ is the angle between the two moments.
It is maximum for the smallest angle, i.e. configuration (c) with $\theta = 30^\circ$.

Let the pole strength be m; then $M = ml$.
After bending into an arc subtending $60^\circ$: $r\left(\frac{\pi}{3}\right) = l \Rightarrow r = \frac{3l}{\pi}$
Distance between the poles $= 2r\sin\frac{60^\circ}{2} = r$
$M' = m\times2\left(\frac{3l}{\pi}\right)\left(\frac{1}{2}\right) = \frac{3ml}{\pi} = \frac{3M}{\pi}$