The effective length of the magnet is
A. The complete length of the magnet
B. The distance between the two poles of the magnet
C. The half of the length of the magnet
D. The square of the length of the magnet

Hint: The length of the magnet is always measured between two poles.
In a bar magnet, the poles of the magnet are situated a bit inside the ends of the magnet, and this length in the magnet is known as the magnetic length. The actual length of the magnet is called the geometric length. The effective length of the magnet is always less than its geometric length.

The effective length of a magnet is the shortest distance between the top and bottom-most points of the column at the point of bending.

Complete step by step answer:
The length of a magnet is defined as the distance between one of the magnetic poles and the center of that magnet.
The effective length of the magnet is the distance between the magnetic north pole and the magnetic south pole in case of a bar magnet. The effective length is twice the length of the magnet.

Effective length= 2l
Actual length=L
Hence we can say the effective length of the magnet is the distance between the two poles of the magnet, and it is less than the geometric length of the magnet.

So, the correct answer is “Option B”.

Note:
Students often confuse the terms effective length and the geometric length, they must note that the geometric length gives end to end length of the magnet and magnetic length gives the length between the poles.