The magnetic susceptibility is
Correct option is D. $$ x = \frac {M}{H} $$
Magnetic susceptibility is a dimensionless proportionality constant that indicates the degree of magnetization of a material in response to an applied magnetic field. It is caused by interactions of electrons and nuclei with the externally applied magnetic field.
The mathematical definition of magnetic susceptibility is the ratio of magnetization to applied magnetizing field intensity. This is a dimensionless quantity. i.e $$X = \dfrac{M}{H}$$