When an organic compound containing sulphur is fused with sodium, the sulphur is converted into sodium sulphide (Na_2S). The sulphide ion in the fusion extract reacts with sodium nitroprusside, Na_2[Fe(CN)_5NO], to produce a deep violet or purple complex, sodium thionitroprusside Na_4[Fe(CN)_5NOS], which is the confirmatory test for sulphur. Option Nitrogen is wrong because nitrogen is detected through the Prussian blue test using ferrous sulphate, not through nitroprusside. Option Chlorine is incorrect since halogens are detected by the silver nitrate test producing characteristic silver halide precipitates. Option Phosphorus is wrong because phosphorus is identified by heating with an oxidising agent and testing for phosphate with ammonium molybdate. The sodium nitroprusside reaction is the NCERT-prescribed qualitative test for sulphide ions derived from organic sulphur. Plausibility check: the violet colour arises specifically from the S^{2-} ion coordinating to the nitroprusside complex, so the test responds only to sulphide and not to halide or cyanide ions, confirming sulphur uniquely.