Group II elements form +2 oxides with 1:1 metal-to-oxygen ratio

Steps:

• Group II elements (alkaline earth metals) have a +2 oxidation state in compounds.
• The oxide ion is O²⁻, requiring a +2 cation for charge balance.
• One Z²⁺ ion pairs with one O²⁻ ion, yielding the formula ZO.
• This matches empirical formulas like MgO or CaO for Group II oxides.

Why A is correct:

• ZO reflects the +2 valence of Group II metals, balancing with divalent oxide per the general formula MO.

Why the others are wrong:

• B. ZO₂ implies +4 oxidation state, typical of Group IV elements like Ti.
• C. Z₂O suggests +1 oxidation state, characteristic of Group I alkali metals like Na.
• D. ZO duplicates A but is listed separately; no distinction in formula.

Final answer: A