d
Conjugate acid base pair - If bronsted acid is a strong acid then its conjugate base is a weak base and vice versa.

\(H C l+H_{2} O \rightleftharpoons H_{3} O^{+}+C l^{-}\)

\(\mathrm{Cl}^{-}\) is conjugate base of \(\mathrm{HCl} \& \mathrm{H}_{3} \mathrm{O}^{+}\) is conjugate acid of \(\mathrm{H}_{2} \mathrm{O}\)

Stronger acids has weaker conjugate base aciding order:

\(R C O O H>H C \equiv C H>N H_{3}>R H\)

So basicity order : \(R C O O^{-}  < H C \equiv C^{-} < N H_{2}^{-} < R^{-}\)