The identification and conformation test of nitrate ion ( \\\\(NO_3^{–}\\\\)) using conc. \\\\(H_2SO_4 and freshly prepared FeSO4 , is called as 'Brown Ring Test. In a test tube take the \\\\)NO_3^{–}\\\\( containing sample .Then add conc.\\\\)H_2SO_4\\\\( dropwise with gentle shaking and then heat it shortly using Bunsen burner & add freshly prepared greenish \\\\)FeSO_4\\\\(. Then, we can observe a brown ring formed in that test tube due to the formation of brown colored \\\\)[Fe(H_2O)5 (NO)]SO_4 complex.Hence option A is correct.