CGPET 2019 Solved Paper

When Fe2+ i.e., [Fe(H2O)]SO4 or FeSO4(aq)
react with NO (nitric oxide), it gives brown ring complex as follows
[Fe(H2O)6]SO4+NO ────▸

[Fe(H2O)5NO]SO4

brown‌complex

+H2O
The brown ring test is used to identify the presence of NO3− ions.
Hence, option (a) is the correct answer.