From the following, the least stable structure is :

Stability Rules:

1. Octet Rule: Structures where all atoms have complete octets are more stable.
2. Number of Bonds: More covalent bonds usually mean more stability.
3. Charge Separation: Neutral structures are more stable than charged ones.
4. Electronegativity: Negative charges are more stable on more electronegative atoms (like (O) or (N)).
5. Adjacent Charges: Structures with like charges on adjacent atoms are highly unstable.

All have same number of $$\sigma$$ (sigma bonds) and $$\pi$$ bonds.
No neutral species. All are charged species.

Option (A)

• The positive charge is part of a conjugated system.
• The double bonds allow resonance delocalization of charge.
• The nitro group (-NO₂) helps in stabilizing the system via resonance. Both negative charges on high electronegative atom - Oxygen.

Conclusion:

Better stabilization due to conjugation and resonance. Most stable.

Option (B)

• Both positive charges next to each other so they repel each other and stability decreases
• The double bonds allow resonance delocalization of charge.

Conclusion:

Least stable.

Option (C)

• Charges are arranged such that resonance delocalization is possible.
• (+ve) charge and (-ve) charge are next to each other. So that stabilizes system. But distance between two positive charges (on Carbon and Nitrogen) are closer compared to D $$\rightarrow$$ they repel.

Conclusion:

More stable than option B due to better charge handling.

Option (D)

• Shows conjugation between double bond and charged centers.
• (+ve) charge and (-ve) charge are next to each other. So that stabilizes system more.
• Opposite charges are little far as compared to option C

Conclusion:

More stable compared to C.

Stability Order :

A > D > C > B