Q.In which of the following, NH3 is not used?

Step 1. Read each option as "is NH3 a reactant/ingredient here?"

Nessler's reagent is an alkaline solution of potassium tetraiodomercurate(II), K2[HgI4], in KOH. It is used to test FOR ammonia (giving a brown iodide of Million's base precipitate/colour with even traces of NH3) — ammonia is what is being detected, not a component used to make the reagent.

Step 2. Check the qualitative-analysis uses of ammonia.

Group III basic radicals (Fe3+, Al3+, Cr3+) are precipitated as hydroxides using NH4Cl + NH4OH (aqueous ammonia) — so ammonia IS used in option (c). Group IV basic radicals (Zn2+, Co2+, Ni2+, Mn2+) are precipitated as sulphides in the presence of NH4OH + NH4Cl (to buffer the solution and control S2- concentration with H2S) — so ammonia IS used in option (b) as well.

Step 3. Check Tollen's reagent.

Tollen's reagent is ammoniacal silver nitrate: AgNO3 solution is first treated with NaOH to precipitate AgOH/Ag2O, and then ammonia is added until the precipitate just redissolves, forming the diamminesilver(I) complex [Ag(NH3)2]+. Ammonia is thus essential to option (d) as well.

Step 4. Conclude.

Only option (a) does not use ammonia as an ingredient — Nessler's reagent instead uses ammonia purely as the substance it is designed to detect.

Recall the analytical-chemistry role of ammonia in each named reagent/scheme and check whether NH3 is a reactant used to make/operate it, or merely the species being detected.

Confusing Nessler's reagent (a reagent FOR detecting NH3) with a reagent MADE FROM NH3, and forgetting that both Group III and Group IV basic-radical precipitation in qualitative analysis rely on NH4OH/NH4Cl buffering.