Paramagnetic substances are those which attract the external magnetic field or which move from a weaker to stronger part of the magnetic field. Paramagnetic substances contain unpaired electrons. Diamagnetic substances are those which repel the external magnetic field or which tend to move away from stronger part to weaker part of the magnetic field. Diamagnetic substances are those which contain all paired electrons. Ferromagnetic substances are those which are attracted very strongly into the applied magnetic field. In ferromagnetic substances, the spins of all unpaired electrons and thus their magnetic moments are aligned in the same direction. Ferromagnetism is an extreme form of paramagnetism. Iron, Cobalt and Nickel are ferromagnetic substances. The magnetic property of a substance is due the spin angular momentum and orbital angular momentum of the electron and can be calculated using the formula

\[\mu_{S + L}= \sqrt{4S(S+1)+L(L+1)}BM\]

where S is the sum of spin quantum numbers of all electrons and L is the sum of orbital angular momentum quantum number. The unit of magnetic moment is Bohr magnetons (BM).

\[1 BM = \frac{eh}{4\pi m_e} = 9.237 × 10^{-21} \text{erg Gauss}^{-1}\]

In many compounds of 3d series metals, the magnetic moment due to orbital movement of electron is neglected as the orbital movement of these electrons is quenched by the surrounding species in compound or solution. The spin only magnetic moment can be calculated by using the spin only formula

\[\mu_s = \sqrt{n(n + 2)}BM\]

Atoms in which the spins of all unpaired electrons are aligned in the same direction when kept in external magnetic field are

ferromagnetic

The correct answer is option 3. ferromagnetic.

Let us delve into each type of magnetism to provide a comprehensive explanation:

1. Paramagnetic:

Paramagnetic materials are those with unpaired electrons. When placed in an external magnetic field, the magnetic moments of these unpaired electrons align with the field, resulting in a weak attraction to the magnetic field. However, the alignment is temporary and disappears when the external magnetic field is removed. Examples of paramagnetic materials include oxygen, copper, and aluminum.

2. Diamagnetic:

Diamagnetic materials are those with all electron spins paired. When placed in an external magnetic field, these materials generate small magnetic fields in the opposite direction to the applied field. This results in a weak repulsion from the magnetic field. Diamagnetic behavior is a fundamental property of all materials and arises from the orbital motion of electrons. Examples of diamagnetic materials include water, gold, and bismuth.

3. Ferromagnetic:

Ferromagnetic materials are those with unpaired electrons, similar to paramagnetic materials. However, in ferromagnetic materials, neighboring magnetic moments align parallel to each other even in the absence of an external magnetic field. This alignment leads to the formation of magnetic domains, regions within the material where the magnetic moments are aligned. When an external magnetic field is applied, these domains grow, resulting in a strong attraction to the magnetic field. Ferromagnetism is observed in materials like iron, cobalt, and nickel.

4. Ferrimagnetic:

Ferrimagnetic materials are similar to ferromagnetic materials in that they have unpaired electrons and exhibit permanent magnetization. However, in ferrimagnetic materials, the magnetic moments of neighboring atoms align in opposite directions, resulting in a net magnetic moment. This net magnetic moment is weaker than that of ferromagnetic materials due to the partial cancellation of magnetic moments. Ferrimagnetic behavior is observed in materials like magnetite \((Fe_3O_4)\) and some rare-earth compounds.

In summary, ferromagnetic materials exhibit permanent magnetization due to the alignment of magnetic moments of neighboring atoms in the same direction, making them strongly attracted to external magnetic fields.