CGPET 2019 Solved Paper

Nessler's reagent is K2HgI4. It is a basic solution of potassium mercuric iodide which gives reddish brown ppt. of millon's base [H2N−HgO−HgI] with NH3, as follows
NH3+2K2HgI4+3KOH────▸

H2N−HgO−HgI

(lodide‌of‌millon′‌s‌base)‌Brown‌ppt.

+7KI+2H2O
Thus, it can give [HgI4]2− ions so, option (d) is the correct answer.