When an organic compound containing sulphur is fused with sodium, the sulphur is converted into sodium sulphide, Na2S. In the sodium fusion extract, sulphide ions can be detected by adding sodium nitroprusside solution, which produces a deep violet (purple) colouration due to the formation of a coloured complex, [Fe(CN)5NOS]4-. This violet colour is a reliable confirmatory test for sulphide ions and hence for sulphur in the original compound. Nitrogen detection follows a different route, giving Prussian blue with ferrous and ferric ions, not a violet colour with nitroprusside, so that option is incorrect. Chlorine is detected as chloride by acidifying with nitric acid and adding silver nitrate to give a white precipitate, which is unrelated to the nitroprusside violet colour. Phosphorus is tested by oxidising it to phosphate and treating with ammonium molybdate to give a yellow precipitate, again different from this observation. The sodium nitroprusside violet test is the standard NCERT confirmatory test for sulphur. Plausibility check: a violet colour with nitroprusside is specific to sulphide ions, so it correctly points to sulphur and not to the other elements.