Which of the following is NOT a source of electrical power?

The choice indicating that magnetic (coil) is not a source of electrical power is correct because a coil itself does not generate electrical power unless it is subject to a change in magnetic flux, which induces an electromotive force (EMF) according to Faraday's law of electromagnetic induction. This induction process requires mechanical movement or the presence of an alternating magnetic field to produce electricity.

In contrast, mechanical power can come from generators that convert mechanical energy into electrical energy. Similarly, chemical energy in batteries is transformed into electrical energy through chemical reactions, and photoelectric sources generate electricity from light through photovoltaic effects. Each of these processes directly translates a form of energy into electrical power without needing an external condition to induce it, unlike a magnetic coil that relies on external factors.