Chemistry - Complete d-block Question with Solution | TestHub

Nessler’s reagent, ${\mathrm{K}}_2\left[{\mathrm{HgI}}_4\right]$ is used to detect ammonia. It is a pale solution that gets darker yellow when some amount of ammonia is added. A brown precipitate may form at a higher concentration. Nessler's reagent can be crystallized from a concentrated solution of mercury iodide with potassium iodide.

So, the correct option is D.