1. We need a process where both happen simultaneously:

   • Bond order increases
   • Paramagnetic changes to diamagnetic

2. Recall the idea from Molecular Orbital Theory

   • Bond order: $\text{B.O.} = \frac{N_b - N_a}{2}$ where $N_b$ = bonding electrons and $N_a$ = antibonding electrons.
   • Paramagnetic: has unpaired electron(s)
   • Diamagnetic: all electrons paired

3. Check each option

Option A: $NO \to NO^+$

(i) For $NO$

• Total electrons = $7 + 8 = 15$
• $NO$ has one unpaired electron, so it is paramagnetic.
• Its bond order is: $\text{B.O.}(NO) = 2.5$

(ii) For $NO^+$

• Total electrons = $14$
• $NO^+$ is isoelectronic with $N_2$
• Hence bond order: $\text{B.O.}(NO^+) = 3$
• All electrons are paired, so it is diamagnetic.

Conclusion for A

• Bond order increases: $2.5 \to 3$
• Paramagnetic $\to$ diamagnetic
• A satisfies both conditions

Option B: $N_2 \to N_2^+$

(i) For $N_2$

• Total electrons = $14$
• Bond order: $\text{B.O.}(N_2) = 3$
• It is diamagnetic.

(ii) For $N_2^+$

• One electron removed from bonding MO
• Bond order becomes: $\text{B.O.}(N_2^+) = 2.5$
• It has one unpaired electron, so paramagnetic.

Conclusion for B

• Bond order decreases
• Diamagnetic $\to$ paramagnetic
• Not correct

Option C: $O_2 \to O_2^+$

(i) For $O_2$

• Bond order: $\text{B.O.}(O_2) = 2$
• It is paramagnetic (two unpaired electrons in $\pi^*$ orbitals)

(ii) For $O_2^+$

• One electron removed from antibonding orbital
• Bond order becomes: $\text{B.O.}(O_2^+) = 2.5$
• But still has one unpaired electron, so it remains paramagnetic.

Conclusion for C

• Bond order increases
• Paramagnetic remains paramagnetic
• Not correct

Option D: $O_2 \to O_2^{2-}$

(i) For $O_2$

• Bond order = 2
• Paramagnetic

(ii) For $O_2^{2-}$

• Two electrons added to antibonding orbitals
• Bond order becomes: $\text{B.O.}(O_2^{2-}) = 1$
• All electrons paired, so diamagnetic

Conclusion for D

• Paramagnetic $\to$ diamagnetic, but bond order decreases
• Not correct

4. Final Answer Only Option A satisfies both conditions.

$\boxed{A:\ NO \to NO^+}$

5. Comparison with stored correct answer

• Stored correct answer: A
• Derived answer: A
• They agree.