b
Let \(m\) be strength of each pole of bar magnet of length \(l .\) Then

\(M=m \times l.........(i)\)

When the bar magnet is bent in the form of an arc as shown in figure Then

\(l=\frac{\pi}{3} \times r=\frac{\pi r}{3}\)

or \(\quad r=\frac{3 l}{\pi}\)

New magnetic dipole moment \(M^{\prime} =m \times 2 r \sin 30^{\circ} \)

\(=m \times 2 \times \frac{3 l}{\pi} \times \frac{1}{2}=\frac{3 m l}{\pi}=\frac{3 M}{\pi}\) (Using \((i)\))