When a coil's magnetizing force is applied to a ferromagnetic material, the flux density in the material is:

The correct answer relates to the interaction between the applied magnetizing force from the coil and the properties of the ferromagnetic material. When a coil generates a magnetizing force (also known as magnetic field strength) and is applied to a ferromagnetic material, the material responds by becoming magnetized. This process significantly enhances the magnetic flux density within the material.

Ferromagnetic materials have high permeability compared to air or vacuum, which allows them to concentrate magnetic field lines more effectively. This means that the flux density, which represents the amount of magnetic field present per unit area in the material, will increase beyond the level produced merely by the coil itself. This enhancement occurs because the ferromagnetic material provides a path with less reluctance for the magnetic flux lines, allowing more lines to pass through it than would be produced by the coil alone.

Hence, the flux density in the ferromagnetic material becomes greater than that produced by the magnetizing force of the coil, reflecting the inherent properties of ferromagnetism where the material becomes a stronger magnet than the applied field alone would suggest. This fundamental principle underpins many applications in magnetic testing and material characterization in non-destructive testing, especially with electromagnetic techniques.