The stability of carbanions in the following :
(i) $\mathrm{RC}={\mathrm{C}}^{-}$ (ii) 

(iii) ${\mathrm{R}}_2\mathrm{C}={\mathrm{CH}}^{-}$ (iv) ${\mathrm{R}}_3\mathrm{C}-{\mathrm{CH}}_2^{-}$ is in the order of 

The stability of carbanions is affected due to resonance,inductive effect and s - character of orbitals. Greater the number of groups having + I group (alkyl group) lesser stable would be the carbanion. Further stability of carbanion decreases with decrease in s-character. Benzene carbanions are stablized due to resonance, hence the correct order is  

The correct order of stability of given carbanion is in the order (I)> (III)> (II)>(IV)