Answer
There are several factors that affect the stability of bases. For this problem, the factors that affect stability are resonance and induction.
Work Step by Step
The CH3OCOO- must be the most stable because it has the most resonance forms (due to it having the most oxygen atoms). The CF3OCO- is the second stable because it has electronegative F atoms that are closer to the negatively charged O. The F atoms are electron-withdrawing and can therefore delocalize and stabilize the electrons. The CF3CH2OCO- comes next because the F atoms in this base is farther away from O-, which weakens the induction effect. The fourth stable base is CH3OCO- since it doesn't have any other electronegative atoms. CH3CH2O- comes last because it has no other resonance forms (the negative charge is less stable).
