Quantum Numbers, X-rays & Other Questions question from NEET 1992 (AIPMT)

Answer: (d)

1. C ($Z=6$): $1s^2\,2s^2\,2p^2$
2. $\text{N}^+$ ($7-1 = 6$ electrons): $1s^2\,2s^2\,2p^2$
3. Other pairs have different electron counts ($\text{Li}^+$: 2 vs $\text{Na}^+$: 10; He: 2 vs $\text{Ne}^+$: 9; H: 1 vs Li: 3).
4. So the answer is (d) C and $\text{N}^+$.