Step 1: Concept:
Ionisation potential (IP) increases across period, decreases down group. Also, IP of cation>IP of neutral atom.
Step 2: Explanation:
Electronic configurations:
N (7): \(1s^2 2s^2 2p^3\) - half-filled stable
O (8): \(1s^2 2s^2 2p^4\) - less stable
O\(^+\) (7 electrons): same as N but higher nuclear charge (Z=8)
Na: \(1s^2 2s^2 2p^6 3s^1\) - low IP
O\(^+\) has highest IP due to higher Z than N and same configuration.
Step 3: Conclusion:
Thus, O\(^+\) has maximum IP.