Answer: The correct answer is "law of poles".
Step-by-step explanation:
Magnetic field lines are the area around the magnet in which the magnetic force can be experienced.
Law of poles: Like poles of the magnets repel each other with the magnetic force of repulsion. Unlike poles of the magnets attract each other with the magnetic force of attraction.
The direction of the magnetic field lines is from north pole to south pole. The strength of the magnetic field varies with the distance from the poles.
The strength of the magnetic field is stronger at the poles. It decreases from the poles.
The strength of the magnetic field does not follow an inverse square law. The gravity field and the electric field follow an inverse square law. The force from the magnet decreases faster than an inverse square law because all magnets have two opposite poles.
The strength of a magnetic field follows law of poles.
Therefore, the correct option is (D).