Introduction:
The B/H ratio, also known as the permeability of a material, is a measure of how easily a material can be magnetized. It is the ratio of the magnetic flux density (B) to the magnetic field strength (H) in a material. The B/H ratio is an important parameter in understanding the magnetic properties of different materials. In this question, we are asked to identify which type of material has a non-constant B/H ratio.

Ferromagnetic Materials:
Ferromagnetic materials are known for their strong and permanent magnetization properties. They exhibit a high B/H ratio, meaning that for a given magnetic field strength, the magnetic flux density is relatively high. However, the B/H ratio is not constant for ferromagnetic materials. It varies depending on the level of magnetization. Initially, when the magnetic field strength is low, the B/H ratio is high, indicating a high degree of magnetization. As the magnetic field strength increases, the material reaches its saturation point, and the B/H ratio decreases. At saturation, the material cannot be further magnetized, and the B/H ratio remains constant.

Diamagnetic Materials:
Diamagnetic materials have a negative magnetic susceptibility, which means they are weakly repelled by a magnetic field. These materials have a low B/H ratio because they do not easily get magnetized. However, the B/H ratio for diamagnetic materials is constant and does not vary with the level of magnetization. This is because the magnetic response of diamagnetic materials is very weak compared to other materials.

Paramagnetic Materials:
Paramagnetic materials have a positive magnetic susceptibility, which means they are weakly attracted to a magnetic field. These materials have a higher B/H ratio compared to diamagnetic materials because they can be magnetized to a greater extent. Like diamagnetic materials, the B/H ratio for paramagnetic materials is also constant and does not vary with the level of magnetization.

Non-magnetic Materials:
Non-magnetic materials, as the name suggests, are materials that do not exhibit any magnetic properties. These materials have a B/H ratio of zero since they cannot be magnetized. Just like diamagnetic and paramagnetic materials, the B/H ratio for non-magnetic materials is constant and does not vary.

Conclusion:
From the above discussion, it is clear that the B/H ratio is not constant for ferromagnetic materials. Ferromagnetic materials have a varying B/H ratio depending on the level of magnetization. The B/H ratio is high at low magnetic field strengths and decreases as the material reaches saturation. Therefore, option 'B' is the correct answer to the question.