The order of stability of the carbanions :(CH3)3C-  (I) ; (CH3)2CH   (II) ;  CH3CH2  (III)  and C6H5CH2  (IV)

Stability Order: Benzyle Carbanion > Alkyl carbonionReason: Due to resonance benzyle carbonion is more stable.                     As the number of +IE groups [alkyl groups] increases,                     electron donation towards the carbanion carbon increases,                    and hence electron density [weight] on the carbanion carbon increases,                    therefore stability of carbanion carbon increases          Hint: As electron density increases,                  weigth on carbon increases,                 strain on carbon increases,                 stability decreases.→ Based on the above rules the stability order of given carbanions.