The brown ring test for the `NO_(3)^(-)` ions is due to the formation of :

Options

A. `FeSO_(4).NO_(2)`
B. `[Fe(SO_(4))_(2) NO] H_(2)O`
C. `[Fe(H_(2)O)_(5) NO]^(2+)`
D. None of these

Explanation

To solve the question regarding the brown ring test for the `NO3^(-)` ions, we will follow these steps:

### Step-by-Step Solution:

1. **Understanding the Brown Ring Test**:
The brown ring test is a qualitative test used to detect the presence of nitrate ions (`NO3^(-)`) in a solution.

2. **Reagents Required**:
The test involves the use of freshly prepared iron(II) sulfate solution (`FeSO4`) and concentrated sulfuric acid (`H2SO4`).

3. **Initial Reaction**:
When iron(II) sulfate is mixed with concentrated sulfuric acid, iron(II) ions (`Fe^(2+)`) are present in the solution. The sulfuric acid helps in the formation of the iron(III) ions (`Fe^(3+)`), but initially, we focus on the iron(II) ions.

4. **Formation of the Brown Complex**:
In the presence of nitrate ions, the iron(II) ions react with the nitrate ions in an acidic medium. The reaction leads to the formation of a complex ion:
\[ \text{Fe}^{2+} + \text{NO}_3^- \rightarrow \text{[Fe(NO)]}^{2+} + \text{H}_2O \]
Here, the `NO` (nitric oxide) is formed, and it coordinates with the iron ion.

5. **Final Complex**:
The resulting complex, `[Fe(NO)]^{2+}`, is a brown-colored complex, which is responsible for the characteristic brown ring observed at the interface of the two layers in the test.

6. **Conclusion**:
Therefore, the brown ring test for `NO3^(-)` ions is due to the formation of the complex `[Fe(NO)]^{2+}`.

### Final Answer:
The brown ring test for the `NO3^(-)` ions is due to the formation of the complex `[Fe(NO)]^{2+}`.

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