(c)
Magnetic field B due to a bar magnet of magnetic moment M at distance r of the point on the axis from its centre is given by
Here, 2l is the length of the magnet.
When the distance of the point where the magnetic field has to be calculated is greater than the length of the magnet, i.e , the bar magnet acts like a magnetic dipole whose magnetic field is
B
Now, l in the denominator can be neglected.
So, the correct option is (c).