In third group of qualitative analysis, ${\mathrm{NH}}_4\mathrm{Cl}$ is added before ${\mathrm{NH}}_4\mathrm{OH}$ so that

${\mathrm{NH}}_4\mathrm{OH}$ is a weak electrolyte and dissociates partially as

${\mathrm{NH}}_4\mathrm{OH}\rightleftharpoons {\mathrm{NH}}_4^{+}+{\mathrm{OH}}^{-}\text{. }$

On adding ${\mathrm{NH}}_4\mathrm{Cl}$ the dissociation of ${\mathrm{NH}}_4\mathrm{OH}$ is suppressed such that $\left[{\mathrm{OH}}^{-}\right]$ is very less and only less soluble hydroxides are precipitated.

Hence choices (a) and (b) are correct while (c) and (d) are incorrect