Define magnetic susceptibility, mention its unit.

Hint:Magnetic susceptibility is the measure of the degree of magnetization of a material in response to an externally applied magnetic field. Use the basic definition to find the relation between the fundamental quantities, thus finding the SI unit of it.

Complete step-by-step answer:
Magnetic susceptibility $(\chi )$mathematically given as the ratio of magnetization (M) of material to the intensity of the applied magnetic field (H), which is given by,
$\chi =\dfrac{M}{H}$
Where,
M is the magnetization of the material in response to the externally applied magnetic field.
H is the intensity of the applied magnetic field.
Some materials have unpaired electrons, when we put these materials in an external magnetic field these spinning unpaired electrons act as a dipole and get arranged in the magnetic field direction. This results in the magnetization of the material.
Magnetic susceptibility is the measure of the degree of magnetization of a material in response to the externally applied magnetic field.
Because, magnetization (M) and magnetic field intensity (H) both have the same units A/m. Thus, magnetic susceptibility is a dimensionless unit.

Note: Based on the response to the externally applied magnetic field, magnetic material can be divided as follow;
(1) Paramagnetic material-
These magnetic materials align with the magnetic field. They have small positive magnetic susceptibility value $\left( \chi >0 \right)$. They are weakly attracted by the magnets with relative permeability from 1.00001-1.003. These materials are temperature dependent.
Alkaline earth metal, Aluminum, Oxygen are examples.
(2) Diamagnetic material-
These magnetic materials align against the magnetic field. They have small negative magnetic susceptibility $\left( \chi <0 \right)$. They are repelled by the magnets and move from stronger fields to weaker fields. These materials are independent of temperature.
Gold, Tin, mercury, and water are its examples.
(3) Ferromagnetic material-
These materials are highly magnetized in the magnetic field, and align with the magnetic field. They have high positive magnetic susceptibility value $\left( \chi \right)$. They are strongly attracted by the magnetic field.
Iron, cobalt, nickel are its examples.