b
(b)Magnetic moment of circular loop carrying current \(M = IA = I\left( {\pi {R^2}} \right) = I\pi {\left( {\frac{L}{{2\pi }}} \right)^2} = \frac{{I{L^2}}}{{4\pi }} \Rightarrow L = \sqrt {\frac{{4\pi M}}{I}} \)