Answer
When electrons are more delocalized, the base is more stable.
Work Step by Step
FSOOO- is more stable than CH3SOOO- because the F atom is electronegative and can help delocalize electrons more than CH3. CH3SOOO- is more stable than CH3SOO- because it has more O atoms (more resonance forms) that can help delocalize electrons. CH3SOO- is more stable than CH3COO- because the S central atom is a lot larger than C atom and can allow more stability of the electrons. CH3COO- is more stable than CH3CONH because O is more electronegative than N and haveing the negative charge on a more electronegative atom is more stable.
