pK_b of NH₃ is 4.74 and, PK_b of A^- , B^- and C^- are 4, 5 and 6 respectively. Aqueous solution of 0.01 M has pH in the increasing order as

Salts (NH₄A, NH₄B, NH₄C) are of weak acid and weak base.

$$\begin{gathered} \mathrm{pH}=7+\frac{{\mathrm{pK}}_{\mathrm{a}}}{2}-\frac{{\mathrm{pK}}_{\mathrm{b}}}{2} \\ \mathrm{Thus}, \mathrm{greater} \mathrm{the} \mathrm{value} \mathrm{of} {\mathrm{pK}}_{\mathrm{a}} \mathrm{of} \mathrm{HA}, \mathrm{greater} \mathrm{the} \mathrm{pH}. \\ {\mathrm{pK}}_{\mathrm{b}}\left({\mathrm{A}}^{-},{\mathrm{B}}^{-},{\mathrm{C}}^{-}\right)\begin{array}{c}{\mathrm{A}}^{-}<{\mathrm{B}}^{-}<{\mathrm{C}}^{-}\\ 5\end{array} \\ {\mathrm{pK}}_{\mathrm{a}}(\mathrm{HA},\mathrm{HB},\mathrm{HC})\underset{8}{\mathrm{H}}\mathrm{C}<\underset{9}{\mathrm{HB}}<\underset{10}{\mathrm{HA}} \\ \text{ Thus, }\mathrm{pH}\left({\mathrm{NH}}_4\mathrm{C}<{\mathrm{NH}}_4\mathrm{B}<{\mathrm{NH}}_4\mathrm{A}\right) \end{gathered}$$