Which of the following is not a form of energy?
a. electrical
c. thermal
b. nuclear
d. magnetic

Electric energy is the energy caused by the movement of the electric charges. The moving electric charges form the kinetic energy, thus electrical is the from of energy.

Thermal energy is the energy which usually comes from the heat.This heat is generated by the movement of tiny particles within an object. Thus, thermal energy is the form of energy.

Nuclear energy is the energy which comes from the splitting atoms in a reactor to heat water into steam and generate electricity. Thus, nuclear energy is the form of energy.

The magnets cannot be used to generate energy, as it is kind of a force which attracts a body from the another body, therefore the magnetic is not the form of energy.

Thus, option D is correct.

it takes about 30 days for it to break down