To a solution of an unknown compound, a little dilute tetraoxosulphate (VI) acid was added with some freshly prepared iron (II) tetraoxosulphate (VI) solution. The brown ring observed after the addition of a stream of concentrated tetraoxosulphate (VI) acid confirmed the presence of

Correct answer: NO (i.e., nitrate, NO⁻₃).

Explanation: The brown-ring test uses freshly prepared FeSO₄ and carefully added conc. H₂SO₄. If nitrate is present, nitrate is reduced to NO, which then forms the brown nitrosyl complex [Fe(H₂O)₅(NO)]²⁺ at the acid–solution interface. The brown ring therefore confirms nitrate in the unknown.

• CO — carbon monoxide does not give the brown-ring test.
• Cl⁻ — chloride does not produce a brown ring with FeSO₄/H₂SO₄.
• SO (intended as sulfate) — sulfate does not give the brown-ring test.
• NO — listed option corresponding to nitrate test; the brown ring specifically indicates the presence of nitrate (NO₃⁻) in the sample.