Chemistry - GOC Question with Solution | TestHub

A is an enolate pair: moving the oxygen lone pair into $\mathrm{C}-\mathrm{O}$ and moving the $\mathrm{C}=\mathrm{C}$ pi electrons onto carbon preserves sigma bonds. $\mathrm{B}$ closes an open allyl framework into a three-membered ring; $\mathrm{C}$ opens a cyclopropane bond, so both change sigma connectivity and are not ordinary resonance contributors under the stated definition. In $\mathrm{D}$, the second nitro drawing gives nitrogen two $\mathrm{N}=\mathrm{O}$ double bonds plus its $\mathrm{N}-\mathrm{C}$ bond, exceeding the octet of a second-row atom. Therefore $\mathrm{B}$, $\mathrm{C}$ and $\mathrm{D}$ are invalid ordinary resonance pairs.