Ce ⁴⁺ is stable. This is because of

Step-by-step solution

The electronic configuration of Ce is Ce ₅₈=[ Xe ] 4 f¹ 5 d¹ 6 s² (predicted) or =[ Xe ] 4 f² 5 d⁰ 6 s² (observed) Ce ⁴⁺=[ Xe ] 4 f⁰ 5 d⁰ 6 s⁰ Since, in +4 oxidation state, all (ie, 4 f, 5 d and 6 s ) orbitals are empty and Ce gains the stable configuration of nearest inert gas, Ce ⁴⁺ is most stable.