Answer:
The right answer is D. permanent.
Explanation:
Permanent magnets refer to those magenets that exert an magnetic field due to their internal structure. That is, they naturally generate a magnetic field.
So, when we talk about magnets we use on the refrigerator, we are talking about magnets that naturally exert a magnetic field, due to their internal composition of the material that are made.
Refrigerator's magnets are not superconducting, or induced. There's no need of any electrical current to produce their magnetic field.
Therefore, the right answer is D.
