The correct formula unit for the ionic compound manganese(II) oxide is:
1 Mn²⁺ ion and 1 O²⁻ ion.
This is because manganese(II) oxide consists of manganese in the +2 oxidation state (Mn²⁺) and oxide ions in the -2 oxidation state (O²⁻). The overall charge of the compound is neutral, so one Mn²⁺ ion balances one O²⁻ ion. Thus, the formula for manganese(II) oxide is MnO.
The other options provided are incorrect based on the oxidation states and the known charges of the ions involved.