The correct option is C [Fe(H2O)5NO]SO4
The formula of the compound formed during brown ring test of nitrate radical is pentaaquanitrosyliron (II) sulphate ([Fe(H2O)5NO]SO4) .
Brown ring test is be performed by adding iron (II) sulfate to the solution. When concentrated sulfuric acid is slowly added, a brown ring is formed on the test tube, which will indicate the presence of the nitrate ion. The test follows several phases. These have been written as balanced chemical equations:
Zn(NO3)2+2H2SO4→2ZnSO4+4HNO3
4FeSO4+2H2SO4→2Fe2(SO4)3+2H2 These two reactions happen simultaneously.
HNO3→2H2O+3O2+4NO
The Nitric Acid decomposes in the intense heat produced by the high concentrate Sulphuric Acid used.
2Fe2(SO4)3+2H2+4NO→4[Fe(H2O)5NO]SO4+2H2SO4 The [Fe(H2O)5NO]SO4 forms a brown ring in the middle of the solution produced by the reaction, making it easy to identify the presence of nitrates in the water.