The compound shown is treated with a strong base, which removes one proton from a carbon atom. Among the carbanions shown, which one is the most stable and is the anion actually formed?

The compound shown is cyclopentadiene. Loss of a proton from its CH${}_2$ group gives the cyclopentadienyl anion, a planar, cyclic, fully conjugated ion with 6 pi electrons, which is aromatic. The two vinylic anions (A and D) are not aromatic and keep the negative charge in an $s{p}^2$ orbital outside the pi system, and the saturated cyclopentyl anion (C) is not even derived from the compound shown by loss of one proton.