$A = NH_4Cl$; $ B = CH_3COONa$; $ C = NH_4OH$; $D = CH_3COOH$
$(i)\, CO_2 + H_2O $ $\rightleftharpoons$ $ H_2CO_3 (ii) NH_3+ H_2O $ $\rightleftharpoons$ $ NH_4OH (iii) HCl + H_2O$ $\rightleftharpoons $ $ Cl^-+ H_3O^+$
$2N{H_3}\, \rightleftharpoons \,N{H_4}^ + \, + \,N{H_2}^ - $