Considering only the inductive effect of alkyl groups (usual solution-phase argument), which of the following carbanions is the most stable?

Alkyl groups are electron releasing (+I) and intensify the negative charge on carbon, destabilising the carbanion. The more alkyl groups, the less stable the ion, so the order is $\mathrm{CH}{}_3{}^{-}>1^{\circ }>2^{\circ }>3^{\circ }$. The methyl carbanion is the most stable.