Which of the following carbanions are more stable than the ethyl carbanion, $\mathrm{CH}{}_3\mathrm{CH}{}_2{}^{-}$?

Stability of the carbanion follows the acidity of its conjugate acid. Ethynide (sp carbon), allyl and benzyl anions (resonance) are all more stable than the sp3 ethyl anion. The tert-butyl anion has three electron-donating alkyl groups and is less stable than the ethyl anion.