Moving Charges And Magnetism

In vertical plane in magnetic meridian both horizontal and vertical components of magnetic field exist. When is magnetic moment of the magnet, and are the horizontal and vertical components of earth's magnetic field and is moment of inertia of magnet about its axis of vibration, then the time-period of magnet is when horizontal component is taken [as in vertical plane in magnetic meridian both and act on the needle]

Given, Also the angle of dip at a place is the angle between the direction of earth's magnetic field and the horizontal in the magnetic meridian at that place

Magnetic force,

The correct answer is option (B): Rotating