Answer :

For a bar magnet with length 2l, and magnetic moment M, the magnetic field on any point in the axis at a distance r from the center can be expressed as,



But, in the case of dipole, the length in-between the poles are very small. So, r> l . Hence the above expression will become,



Or,



Hence, we can see that, in the case of dipole, the magnetic field in inversely proportional to the third power of r, that is



Thus, option C is the correct option.

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