Answer is: O (oxygen).
1) Oxygen is an element (nonmetal) with atomic number 8 (8 protons and 8 electrons). Oxygen can form anion (O²⁻) when it gain two electrons to have stable electron configuration like closest noble gas neon (Ne) with atomic number 10.
Electron configuration of oxygen atom: ₈O 1s² 2s² 2p⁴.
Electron configuration of oxygen anion: ₈O²⁻ 1s² 2s² 2p⁶.
2) Potassium (K) and aluminium (Al) are metals, which means they lost electrons, so they form cations, not anions. Potassium form cation K⁺ and aluminium Al³⁺.
3) Argon (Ar) is noble gas, which means it has stable electron configuration, so does not form anions and cations.
