Bar Magnet & Magnetic Moment question from NEET 2013

Answer: (d)

1. Pole strength $m = M/l$ is unchanged.
2. Arc of length $l$ subtends $60^\circ = \pi/3$: $r\cdot\dfrac{\pi}{3} = l \Rightarrow r = \dfrac{3l}{\pi}$.
3. The two radii and the chord form an equilateral triangle, so chord $= r = \dfrac{3l}{\pi}$.
4. $M' = \dfrac{M}{l}\cdot\dfrac{3l}{\pi} = \dfrac{3M}{\pi}$
5. So the answer is (d) $\frac{3}{\pi}M$.