Explanation: The ratio of magnetic flux density (B) to the magnetic field strength (H) is known as **permeability** ($\mu$). Permeability is a fundamental property of a material that quantifies its ability to support the formation of a magnetic field within itself. It essentially describes how easily magnetic lines of force can pass through a material and how much magnetic flux a material will concentrate for a given magnetizing force. Materials with high permeability, like iron, are used in inductors and transformers to concentrate magnetic fields. **Residual magnetism** is the magnetism remaining in a material after the magnetizing force has been removed. **Reluctance** is the opposition that a magnetic circuit offers to the magnetic flux, analogous to resistance in an electrical circuit. Neither of these describes the ratio of magnetic flux density to field strength.