The complex that dissolves in water is :

Solution

Chemistry ma’am · 3 steps

Think salt analysis: three are precipitates, only the iron acetate complex stays as a red solution.

1. Where they come from. All four complexes come from qualitative salt analysis. Ask what you actually see in the test tube when each one forms: a solid that settles, or a coloured solution.

   Complex	Test it belongs to	What you see
   Fe₄[Fe(CN)₆]₃ (Prussian blue)	Fe³⁺ with K₄[Fe(CN)₆]	blue precipitate
   (NH₄)₃[As(Mo₃O₁₀)₄]	arsenate with ammonium molybdate and HNO₃	yellow precipitate
   K₃[Co(NO₂)₆]	K⁺ with sodium cobaltinitrite	yellow precipitate
   [Fe₃(OH)₂(OAc)₆]Cl	acetate with neutral FeCl₃	deep red solution

2. The acetate test. Neutral FeCl₃ added to an acetate gives a deep red colour. The colour is the dissolved cation [Fe₃(OH)₂(CH₃COO)₆]⁺. Only on boiling does it break down into a brown-red precipitate of basic iron(III) acetate.
3. So. The other three are recognised by their insoluble precipitates. Only [Fe₃(OH)₂(OAc)₆]Cl is water-soluble, so the answer is (d).

The idea to keep. Complexes used as confirmatory tests are usually recognised because they precipitate. When a test gives a colour in solution instead, that complex is the soluble one.

Answer from the NTA final answer key. Solution by PrepWiser.