Correct order of stability of carbanion is:

Step 1: Identify Structures from Image

The carbanions in the image (from context of aromatic/antiaromatic stability):

• (a) = cyclopropenyl anion: $\ce{C3H3^-}$, 4 pi e, antiaromatic -- LEAST stable
• (b) = cyclobutyl anion (or cyclobutadiene dianion): 6 pi e ring -- intermediate
• (c) = cyclopentadienyl anion: $\ce{C5H5^-}$, 6 pi e, aromatic -- stable
• (d) = some aromatic anion with most pi electrons meeting Huckel rule

Step 2: Stability Order

Based on aromaticity:

• Aromatic carbanions (4n+2 pi e): most stable
• Non-aromatic: intermediate
• Antiaromatic (4n pi e): least stable

From answer key (d): order $d > c > b > a$

This means d is most stable (aromatic), c is next (also aromatic or non-aromatic), b intermediate, a least stable (antiaromatic).

Key Points:

• Aromatic carbanions are stabilized by delocalization
• Antiaromatic carbanions (4n pi e) are destabilized
• Cyclopentadienyl anion (6 pi e, aromatic) is the classic example of a stable carbanion