Answer:
D) a magnet or magnetic field.
Explanation:
Electricity can be generated from mechanical energy due to a phenomenon called electromagnetic induction.
This phenomenon occurs when the magnetic flux (due to the presence of a magnetic field) through an enclosed circuit is changed: when this happens, an electromotive force is induced in the circuit, and as a consequence, an electrical current is induced in the circuit.
This phenomenon can be produced in different ways; for example:
1- By rotating a coil in a fixed magnetic field --> by rotating the coil, the magnetic flux through the circuit changes over time, so a current is induced in the circuit
2- By varying the intensity of the magnetic field over time --> this way, the magnetic flux through the circuit changes as well
Number #1 is used in electrical generators to produce electrical energy from mechanical energy (kinetic energy): in fact, a coil is made rotating (so, kinetic energy) in a magnetic field, producing an electrical current (electrical energy). It is clear that an essential element of this system is the presence of a magnet or a magnetic field, so D) is the correct answer.
