Magnetic moment of a thin bar magnet is ’M’. If it is bent into a semicircular form, its new magnetic moment will be ________ .

The correct option is M.

Step-by-step Explanation:

1. Definition of Magnetic Moment in terms of volume:
The magnetic moment of a magnet can be described macroscopically as the product of its magnetization $I$ (magnetic moment per unit volume) and its total volume $V$:
$M=I\times V$

2. Analyzing the physical changes:
When the thin bar magnet is bent into a semicircular form, we observe the following:
- The total amount of magnetic material remains the same, meaning the volume $V$ is conserved.
- The intrinsic alignment of the magnetic domains (the magnetization $I$) remains unchanged as the material is only geometrically reshaped without altering its magnetic properties.

3. Conclusion:
Since both the volume $V$ and the magnetization $I$ remain constant during the deformation, the total magnetic moment of the magnet is conserved.
Consequently, the new magnetic moment is equal to the original value, which is M.