A bar magnet of magnetic moment M and moment of inertia I is freely suspended such that the magnetic axial line in the direction of magnetic meridian. If the magnet is displaced by a very small angle ($\theta$), the angular acceleration is (Magnetic induction of earth's horizontal field$=B_H$)

Question

$$\begin {array} {1 1} (a)\;\frac{MB_H \theta}{I} & \quad (b)\;\frac{IB_H \theta}{M} \\ (c)\;\frac{M \theta}{IB_H} & \quad (d)\;\frac{I \theta}{MB_H} \end {array}$$