The correct answer is: A. Electrons
In a metal wire, electric current is due to the flow of electrons. Electrons are negatively charged particles that are free to move around in the metal. When a voltage is applied to the wire, the electrons are attracted to the positive terminal and repelled by the negative terminal. This causes them to move through the wire, creating an electric current.
Protons are positively charged particles that are found in the nucleus of an atom. Ions are atoms or molecules that have a net electrical charge, either positive or negative. Holes are regions in a semiconductor that have a positive charge, due to the absence of an electron.
Protons, ions, and holes can all carry charge, but they are not the main carriers of electric current in a metal wire. Electrons are the only particles that are free to move around in a metal, and they are the ones that create the electric current.