Calcium ion has three complete electron shells Steps:

• Identify electron configuration of O²⁻: 1s² 2s² 2p⁶ (10 electrons, two complete shells: n=1 and n=2).
• Note the question likely seeks an element forming a 2+ ion with three complete shells (n=1,2,3 filled), analogous to O²⁻ but extended.
• For each element, determine the 2+ ion configuration: requires 18 electrons for [Ar]-like structure (2+8+8).
• Calcium (atomic number 20) loses 2 electrons to form Ca²⁺ with 18 electrons and three complete shells.

Why A is correct:

• Ca²⁺ has the electron configuration [Ar] (1s² 2s² 2p⁶ 3s² 3p⁶), filling three shells completely per the octet rule for noble gas stability.

Why the others are wrong:

• B. Mg²⁺ has 10 electrons ([Ne]), only two complete shells.
• C. Oxygen forms O²⁻ (10 electrons, two shells), not a 2+ ion.
• D. Sulfur typically forms S²⁻ (18 electrons, three shells) or higher oxidation states, not a stable 2+ ion with three shells.

Final answer: A