A

To determine the time period of oscillation of a freely suspended bar magnet, we apply the principles of magnetic torque and oscillations. The governing formula for the time period (T) of a suspended magnet is derived from the relationship between the magnetic moment (M) and the magnetic field (H) in which the magnet is oscillating.

The standard formula is given by:
T = 2\pi \sqrt{\frac{I}{MBH}}. Here, I represents the moment of inertia, M represents the magnetic moment of the bar magnet, and H represents the magnetic field strength.

This matches with the expression provided in Option A. Therefore, the correct answer is A.