Which one of the following phrasers best describes flux density? A. The concentration of lines of magnetic force B. The rate of change of the number of lines of magnetic force C. The opposition to a build-up of magnetic fields D. The force that maintains lines of force E. None of the above

The correct answer is: A. The concentration of lines of magnetic force.

Magnetic flux density is a measure of the strength of a magnetic field at a given point. It is defined as the number of magnetic field lines passing through a unit area perpendicular to the field lines. The greater the magnetic flux density, the stronger the magnetic field.

Option B is incorrect because it describes the rate of change of magnetic flux density, not magnetic flux density itself. Option C is incorrect because it describes the reluctance of a material to a magnetic field, not magnetic flux density itself. Option D is incorrect because it describes the force that causes magnetic field lines to exist, not magnetic flux density itself. Option E is incorrect because it is not one of the possible answers.