What is the reason for the magnetism in a magnet?
$\left( A \right)$ The orbital motion of the electrons.
$\left( B \right)$ The spin motion of the electrons.
$\left( C \right)$ The nuclear charge.
$\left( D \right)$ None of the above.

Hint: Magnetism is the force exerted by magnets when they attract or repel each other. Electrons in most atoms exist in pairs. And it happens because of the motion in the electric charges. So by using the hint, now we can easily answer this question.

Complete Step by Step Solution: Around each magnet, there is an unseen region that would attract any magnetic object. This unseen region is known as the magnetic field. Magnetism is caused by electrons spinning. To produce a strong magnetic force, more electrons should spin in the same direction. And in almost all the materials, the magnetic field of ${\text{1electron}}$ is being void by contrasting a magnetic field which is formed by the other pair of electrons.
From the above point, it is clear that the magnetism in a magnet is caused by the spin motion of electrons.

Hence, the option $\left( b \right)$ is correct.

Additional information: Magnets can attract or repel depending on the material and direction of spin.
Objects that are greatly attracted by magnets have iron in them, this is called ferromagnetism.
Objects that are weakly attracted by magnets have aluminum in them, this is called paramagnetism.
And few metals are diamagnetic, meaning they weakly repel magnets.
There is a kind of magnet which only works when electricity is running through it. Here, the electric current produces the magnetic field.
The Earth itself has an electric field, due to changes in the earth’s core, the North Pole moves over time.

Note: An applied orbital motion can cause a change in the orbital motion of electrons, but orbital motion of the electron cannot cause magnetism. Magnets are useful in many ways, magnets are being used in compass, and they are also a very important component in MRI (magnetic resonance imagers).