The correct decreasing order of the basic strength is

According to the question, Conjugate acids and bases are part of the Bronsted-Lowry theory of acids and bases.

According to this theory, the species that donates a hydrogen cation or proton in a reaction is a conjugate acid, while the remaining portion or the one that accepts a proton or hydrogen is the conjugate base.

The conjugate base may be recognised as an anion.

Conjugated base of weak acid is worked as strong base.

$$\begin{gathered} {{\mathrm{CH}}_3}^{-} + {\mathrm{H}}^{+}\to {\mathrm{CH}}_4 (\mathrm{W}.\mathrm{A}) \\ {\mathrm{CN}}^{-}+{\mathrm{H}}^{+}\to \mathrm{HCN} \\ {{\mathrm{NH}}_2}^{-}+{\mathrm{H}}^{+}\to {\mathrm{NH}}_3 \\ {\mathrm{HS}}^{-}+{\mathrm{H}}^{+} \to {\mathrm{H}}_2\mathrm{S} (\mathrm{S}.\mathrm{A}) \end{gathered}$$