Order of increasing stability of the following carbanions is: ${\left({\mathrm{CH}}_3\right)}_3{\mathrm{C}}^{-}\left(\mathrm{I}\right),{\left({\mathrm{CH}}_3\right)}_2{\mathrm{CH}}^{-}\text{(II), }$ ${\mathrm{CH}}_3{\mathrm{CH}}_2^{-}\text{(III), }{\mathrm{C}}_6{\mathrm{H}}_5{\mathrm{CH}}_2^{-}\left(\mathrm{IV}\right),$

Benzyl carbanions are more stable than simple primary alkyl carbanions due to
resonance. As the number of alkyl groups increase, the intensity of the negative
charge on the central carbon atom increase and hence the stability decreases
accordingly.