Basic idea

Basic strength of a conjugate base is inversely proportional to the acidity of its conjugate acid.

So, compare the conjugate acids of the given bases:

• $\ce{OH^-}$ has conjugate acid $\ce{H2O}$
• $\ce{RO^-}$ has conjugate acid $\ce{ROH}$
• $\ce{CH3COO^-}$ has conjugate acid $\ce{CH3COOH}$
• $\ce{Cl^-}$ has conjugate acid $\ce{HCl}$

Compare conjugate acids

Their approximate acid strengths are:

• $\ce{ROH}$ is a very weak acid
• $\ce{H2O}$ is also a very weak acid, but alcohols are generally less acidic than water
• $\ce{CH3COOH}$ is a weak acid, much stronger than water/alcohol
• $\ce{HCl}$ is a strong acid

Hence acidity order of conjugate acids is: $\ce{HCl > CH3COOH > H2O > ROH}$

Therefore, reverse order for basic strength of conjugate bases

Reverse the above order: $\ce{RO^- > OH^- > CH3COO^- > Cl^-}$

Why this makes sense

• $\ce{RO^-}$ is stronger base than $\ce{OH^-}$ because alkyl group shows $+I$ effect and destabilizes the negative charge slightly, making it more eager to accept a proton.
• $\ce{CH3COO^-}$ is weaker because the negative charge is stabilized by resonance.
• $\ce{Cl^-}$ is the weakest base because its conjugate acid $\ce{HCl}$ is very strong.

Match with options

The correct decreasing order is: $\ce{RO^- > OH^- > CH3COO^- > Cl^-}$

This matches Option D.