An organic reaction occurs through making and breaking of bonds. The breaking of bonds may occur either homolytically leading to the formation of radicals or heterolytically generating positively and negatively charged species. The neutral species (free radicals, carbenes, nitrenes, etc.) and positively charged species being electron deficient are collectively called electrophiles while neutral and negatively charged species which are electron rich are called nucleophiles. An organic reaction usually involves the attack of a reagent (radicals, positively and negatively charged species) on the substrate molecule). The substrate molecule, although as a whole electrically neutral, has centers of low and high electron density due to displacement of bonding electrons. These electron displacements occur through inductive, electromeric, resonance and hyperconjugation effects. Whereas inductive effects involve displacement $\mathrm{\sigma }$-electrons, resonance effects involve transfer of $\mathrm{n}$- and $\mathrm{\pi }$-electrons along a conjugated system. Hyperconjugatlon effects, on the other hand, involve $\mathrm{\sigma }$-$\mathrm{\pi }$-conjugation. Both inductive and hyperconjugation effects can be used to explain the stability of carbocations and free radicals which follow the stabilIty order : $3°>2°>1°$. The stability of carbanions, however, follows opposite order.
The stability of a molecule can be judged on the basis of contribution of its resonance structures. Resonance structures have same position of nuclei and have the same number of unpaired electrons. Among resonance structures, the one which has greater number of covalent bonds, has less separation of opposite charges, a negative charge on more electronegative and a positive charge on a more electropositive atom are more stable than others.
The most stable carbanion among the following: