From the following, the least stable structure is:

Solution

Chemistry ma’am · 3 steps

Look at what sits right next to NO₂: a carbanion there is helped, a carbocation there is badly hurt.

1. What to check. All four are charge-separated resonance structures of the same nitrodiene. Check where each charge lands. NO₂ pulls electrons strongly by -I and -R, and its N already carries a formal positive charge.
2. The rule. A negative charge next to NO₂, or on an oxygen, is stabilised. A positive charge on the carbon bonded to NO₂ is destabilised. That carbon is already electron-poor, and its C⁺ would sit beside the positive N.

   Structure	Where the + sits	Where the − sits	Verdict
   (a)	C3, allylic to the C=C, so it is delocalised	C4, on the carbon bonded to NO₂, so it is stabilised	charges placed the right way round
   (b)	C1, a primary carbon	C2, conjugated through C=C to NO₂	− still reaches NO₂
   (c)	C1, conjugated with the C=C	on oxygen, the most electronegative atom	acceptable contributor
   (d)	C4, bonded directly to NO₂	C3, away from NO₂	+ beside the N⁺ of the nitro group, very unstable

3. So. Structure (d) puts the carbocation on the very carbon that NO₂ drains, next to a positive nitrogen. It is the least stable structure.

The idea to keep. In charge-separated structures, a good contributor keeps negative charge near electron-withdrawing groups or on electronegative atoms. A positive charge beside NO₂ is the worst placement.

Answer from the NTA final answer key. Solution by PrepWiser.