The order of stability of the carbanions :

${\left({\mathrm{CH}}_3\right)}_3{\mathrm{C}}^{-} \left(\mathrm{I}\right) ; {\left({\mathrm{CH}}_3\right)}_2\overline{\mathrm{C}}\mathrm{H} \left(\mathrm{II}\right) ; {\mathrm{CH}}_3\overline{\mathrm{C}}{\mathrm{H}}_2 \left(\mathrm{III}\right) \mathrm{and} {\mathrm{C}}_6{\mathrm{H}}_5\overline{\mathrm{C}}{\mathrm{H}}_2 \left(\mathrm{IV}\right)$

Stability Order: Benzyle Carbanion > Alkyl carbonion

Reason: 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.