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

• A
  $\frac{M B_H \theta}{I}$
• B
  $\frac{I B_H \theta}{M}$
• C
  $\frac{M \theta}{I B_H}$
• D
  $\frac{I \theta}{M B_H}$