Definition of Magnetic Susceptibility
Magnetic susceptibility is a property of materials that describes their response to an applied magnetic field. It is a dimensionless quantity that measures the extent to which a material can be magnetized in the presence of an external magnetic field.

Diamagnetic Materials
Diamagnetic materials have a negative magnetic susceptibility, meaning that they weakly repel magnetic fields. When a diamagnetic material is placed in an external magnetic field, it generates a magnetic field in the opposite direction, causing a slight repulsion.

Paramagnetic Materials
Paramagnetic materials have a positive magnetic susceptibility, meaning that they weakly attract magnetic fields. When a paramagnetic material is placed in an external magnetic field, its atomic or molecular magnetic moments align with the field, resulting in a weak attraction.

Ferromagnetic Materials
Ferromagnetic materials have a very large positive magnetic susceptibility, meaning that they strongly attract magnetic fields. They exhibit spontaneous magnetization and can retain a significant amount of magnetization even after the external field is removed. Ferromagnetic materials include iron, nickel, and cobalt.

Ferrimagnetic Materials
Ferrimagnetic materials have a magnetic susceptibility that is less than zero but greater than that of diamagnetic materials. They exhibit a spontaneous magnetization similar to ferromagnetic materials but have two sublattices with opposing magnetizations, resulting in a net magnetization.

Explanation of the Correct Answer
The correct answer is option (c) - Diamagnetic. Diamagnetic materials have a negative magnetic susceptibility, which is less than zero. As mentioned earlier, diamagnetic materials weakly repel magnetic fields and generate a magnetic field in the opposite direction when placed in an external magnetic field. This behavior is a result of the induced currents in the material that oppose the applied magnetic field.

Therefore, if the magnetic susceptibility of any material is less than zero, it indicates that the material is diamagnetic and exhibits weak repulsion in the presence of an external magnetic field.