In Lassaigne's test, the organic compound is fused with sodium metal, converting any nitrogen present into sodium cyanide (NaCN). When the sodium fusion extract is treated with freshly prepared ferrous sulphate and then warmed, the cyanide ions react with Fe^{2+} to form sodium hexacyanoferrate(II), Na_4[Fe(CN)_6]. On acidifying with dilute sulphuric acid, some Fe^{2+} is oxidised to Fe^{3+}, which reacts with the hexacyanoferrate(II) ion to give ferric ferrocyanide, Fe_4[Fe(CN)_6]_3, the intense Prussian blue precipitate that confirms nitrogen. Option Fe(SCN)_3 is wrong because blood-red ferric thiocyanate forms only when both nitrogen and sulphur are present together. Option Na_4[Fe(CN)_6] is the intermediate, which is colourless to pale yellow and not the final blue species. Option AgCN forms in silver nitrate tests, not in the iron-based nitrogen test. This procedure is the standard NCERT method for qualitative element detection. Plausibility check: the deep blue colour requires both Fe^{2+} (in the complex) and Fe^{3+} (the counter-cation), consistent with the mixed-oxidation-state formula Fe_4[Fe(CN)_6]_3.