Ammonium salts on heating with slaked lime liberates a colourless gas(X). Identify the correct statement for gas (X). 

Ammonium salts on heating with slaked lime liberates a colourless gas(${\mathrm{NH}}_3$)

${\mathrm{NH}}_4\mathrm{Cl}+\mathrm{Ca}(\mathrm{OH}{)}_2\to {\mathrm{CaCl}}_2+{\mathrm{H}}_2\mathrm{O}+{\mathrm{NH}}_3\uparrow$

Here gas X is ${\mathrm{NH}}_3$

$\Rightarrow {\mathrm{NH}}_3$ is alkaline in nature hence turns red litmus blue ${\mathrm{NH}}_3+\mathrm{HCl}⟶{\mathrm{NH}}_4\mathrm{Cl}$ giving white fumes 

$\begin{array}{l}\Rightarrow 2{\mathrm{NH}}_3+2{\mathrm{HgNO}}_3\to {\mathrm{Hg}}_3\left({\mathrm{NH}}_2{)\mathrm{NO}}_3+{\mathrm{Hg}}^2+{\mathrm{NH}}_4{\mathrm{NO}}_3\right.\\ {\mathrm{CuSO}}_4+4{\mathrm{NH}}_3⟶\left[\mathrm{Cu}{\left(\frac{\text{ black }}{{\left({\mathrm{NH}}_3\right)}_4}\right]}^{{\mathrm{SO}}_4}\right.\\ \text{ deep blue }\end{array}$

$\Rightarrow 2{\mathrm{K}}_2\left({\mathrm{HgI}}_4\right)+{\mathrm{NH}}_3+3\mathrm{KOH}\to \underset{\left(\mathrm{Brown}\right)}{{\mathrm{HgOHgNH}}_2\mathrm{I}}\downarrow +7\mathrm{KI}+2{\mathrm{H}}_2\mathrm{O}$

The correct option is D.