Identify the test being described

The question refers to the Lassaigne's test using sodium fusion extract.

On treating the sodium fusion extract with $\mathrm{FeSO_4}$ followed by acidification with concentrated $\mathrm{H_2SO_4}$:

• Prussian blue colour indicates nitrogen.
• Blood red colour indicates formation of thiocyanate-related complex, which occurs when both nitrogen and sulfur are present.

Relevant reactions

During sodium fusion, if both $\mathrm{N}$ and $\mathrm{S}$ are present in the organic compound, they combine with sodium to form: $\mathrm{Na + C + N + S \rightarrow NaSCN}$

On treatment with $\mathrm{Fe^{3+}}$ (generated in the test), thiocyanate gives blood red ferric thiocyanate complex: $\mathrm{Fe^{3+} + SCN^- \rightarrow [Fe(SCN)]^{2+}}$

This complex produces the blood red colour.

Evaluate the options

• A: Br — Incorrect. Bromine is tested differently, not by blood red colour with this reagent sequence.
• B: S — Incorrect. Sulfur alone gives tests like violet with sodium nitroprusside or black precipitate with lead acetate, not this blood red colour.
• C: N and S — Correct. This gives $\mathrm{SCN^-}$ in sodium fusion and hence blood red colour.
• D: N — Incorrect. Nitrogen alone gives Prussian blue, not blood red.

Final answer

The blood red colour indicates the presence of: $\boxed{\text{N and S}}$