The correct answer is option 'D' - Susceptibility.

Susceptibility is the parameter used to assess the magnetic ability of a material. It measures the extent to which a material can be magnetized in the presence of an external magnetic field. In other words, it quantifies the material's ability to become magnetized when subjected to a magnetic field.

Susceptibility can be defined as the ratio of the magnetization of a material to the applied magnetic field. It is denoted by the symbol χ (chi). The formula for susceptibility is:

χ = M/H

Where:
- χ is the susceptibility
- M is the magnetization of the material
- H is the applied magnetic field

Susceptibility can be positive or negative depending on the material's response to the magnetic field.

- Paramagnetic materials have positive susceptibility, indicating that they are weakly attracted to a magnetic field. These materials have unpaired electrons that align with the external magnetic field, resulting in a weak magnetization. Examples of paramagnetic materials include aluminum, platinum, and oxygen.

- Diamagnetic materials have negative susceptibility, indicating that they are weakly repelled by a magnetic field. These materials have all their electrons paired, so they do not have a net magnetic moment. Examples of diamagnetic materials include water, copper, and gold.

- Ferromagnetic materials have a very high positive susceptibility, indicating that they can be strongly magnetized. These materials have domains, which are regions of aligned magnetic moments. When subjected to an external magnetic field, these domains align in the same direction, resulting in a strong magnetization. Examples of ferromagnetic materials include iron, nickel, and cobalt.

Susceptibility is an important parameter in the study of magnetism as it helps classify materials and understand their magnetic behavior. By measuring the susceptibility of a material, scientists and engineers can determine its magnetic properties and its potential applications in various fields such as electronics, telecommunications, and medicine.