A brown ring is formed in the ring test for NO3- ion. It is due to the formation of-

1.  Fe H2O5 NO2+2.  FeSO4. NO23.  Fe H2O4 NO22+4.  FeSO4. HNO3

Hint: The IUPAC name of the compound formed in brown ring is PentaaquanitrosylIron(I)Sulphate.

The brown ring complex compound is given as
The complex ion is [Fe(H2O)5NO]2+
Oxidation numbers of H2O is zero and NO is +1
Oxidation state of Fe is +1.

The IUPAC name of the compound is PentaaquanitrosylIron(I ) Sulphate.