Answer: hydrogen
Explanation: Hydrogen is usually considered an energy carrier, like electricity, as it must be produced from a primary energy source such as solar energy, biomass, electricity (e.g. in the form of solar PV or via wind turbines), or hydrocarbons such as natural gas or coal. On the other hand, magnesium, carbon, and iodine are not often used as conductors of electricity because they either have a covalent bond that does not generate electricity or because they contain particles or compounds that can’t be installed inside electric installations.