The most stable carbanion among the following is

Structure A: The carbanion is not the most stable, because it hasn't a resonance effect. 

Structure B: The carbanion is stable due to the benzene ring has a resonance effect. 

Structure C: $-{\mathrm{OCH}}_3$ has a +M effect and its increased electron density makes the carbocation more stable.

Structure D:  $-{\mathrm{NO}}_2$ shows the -M effect and hence it decreases the electron density of benzene rings and due to this event benzene rings goes more stable. 

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