The hydrocarbon shown is for reference. Which of the carbanions shown is the most stable?

AStructure A
BStructure B
CStructure C
DStructure D

Answer: C

Stability of a carbanion increases with the s-character of the carbon bearing the lone pair. The ethynide anion (sp, 50% s) is most stable, ahead of the vinyl anion (sp2, 33%), then the alkyl anions (sp3, 25%), and the tertiary alkyl anion is the least stable.

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