O²⁻, F⁻, Al, Mg²⁺, Na⁺, O⁺, Mg, Al³⁺, F

Correct Answer: 5

Isoelectronic species have the same number of electrons.

Number of electrons:

• O²⁻ → 8 + 2 = 10
• F⁻ → 9 + 1 = 10
• Na⁺ → 11 − 1 = 10
• Mg²⁺ → 12 − 2 = 10
• Al³⁺ → 13 − 3 = 10

These five species are isoelectronic.

Others have different number of electrons and are not isoelectronic.

Hence, total number of isoelectronic species = 5.