An aqueous solution of compound A gives white precipitate with 2 M HCI. The precipitate becomes black on addition of aqueous ${\mathrm{NH}}_3$ due to formation of B.B dissolves in aquaregia A and B are:

$\underset{\text{ (A) }}{{\mathrm{Hg}}_2^{2+}}\stackrel{\mathrm{HCl}}{⟶}\underset{\text{ (White ppt.) }}{{\mathrm{Hg}}_2{\mathrm{Cl}}_2}$$\downarrow$ $\stackrel{\text{ aq. }{\mathrm{NH}}_3}{\to }$ $\underset{\mathrm{Black}\left(B\right)}{\underbrace{\mathrm{Hg}+{\mathrm{Hg}}_2\left({\mathrm{NH}}_2\right)\mathrm{Cl}\downarrow }}$

Since Hg gives white precipitate when treated with HCl. Then white ppt. turn into black by dissolving in aq.${\mathrm{NH}}_3$. The correct option is D.