(a) The magnetism of the magnetic is due to the spin motion of electrons. Each electron in an atom is revolving in an orbit around the nucleus. The revolving electron is equivalent to a tiny loop of current. Therefore, it possesses some orbital magnetic dipole moment $\vec{M}_{1}=$ current $\times$ area of the loop.

In addition to the orbital motion, every electron is assumed to have a spin motion around its axis. Therefore, another dipole magnetic moment called spin magnet $\vec{M}_{s}$ is also associated with electron. The vector sum of $\vec{M}_{l}$ and $\vec{M}_{s}$ provides the net magnetic dipole moment $\vec{M},$ to the atom. $\vec{M}_{s}$ is much greater than $\vec{M}_{l}$, hence magnetism of magnet is due to spin of electrons.