The prussian blue colour obtained during the test of nitrogen by lassaigne's test is due to the formation of:

Options

A. `Fe[Fe(CN)_6]_3`
B. `Fe_2[Fe(CN)_6]`
C. `Fe_3[Fe(CN__6]_4`
D. `Na_4[Fe(CN)_6]`

Explanation

To solve the question regarding the Prussian blue color obtained during the test of nitrogen by Lassaigne's test, we can break down the process step by step:

### Step-by-Step Solution:

1. **Understanding Lassaigne's Test**:
- Lassaigne's test is a qualitative analysis method used to detect the presence of nitrogen in organic compounds. It involves fusing the organic compound with sodium metal to form sodium cyanide.

**Hint**: Remember that Lassaigne's test is specifically for detecting nitrogen.

2. **Formation of Sodium Cyanide**:
- When the organic compound is fused with sodium, sodium cyanide (NaCN) is formed if nitrogen is present in the compound.

**Hint**: Think about the reaction between sodium and nitrogen-containing organic compounds.

3. **Boiling with Iron(II) Sulfate**:
- The sodium cyanide solution is then boiled with iron(II) sulfate (FeSO₄). The iron(II) ions (Fe²⁺) present in the solution will react with the cyanide ions (CN⁻) from sodium cyanide.

**Hint**: Focus on the interaction between iron(II) ions and cyanide ions.

4. **Formation of Hexacyanoferrate(II)**:
- This reaction leads to the formation of sodium hexacyanoferrate(II) (Na₂[Fe(CN)₆]), which is a complex ion.

**Hint**: Remember that this complex is crucial for the next steps.

5. **Acidification and Oxidation**:
- The solution is then acidified with concentrated sulfuric acid (H₂SO₄). During this process, some of the Fe²⁺ ions are oxidized to Fe³⁺ ions.

**Hint**: Consider the role of sulfuric acid in facilitating oxidation.

6. **Formation of Prussian Blue**:
- The Fe³⁺ ions then react with the sodium hexacyanoferrate(II) to form a complex known as iron(III) hexacyanoferrate(II), commonly referred to as Prussian blue (Fe₄[Fe(CN)₆]₃).

**Hint**: Identify the final complex that gives the characteristic blue color.

7. **Conclusion**:
- The formation of the complex Fe₄[Fe(CN)₆]₃ is responsible for the Prussian blue color, confirming the presence of nitrogen in the original organic compound.

**Hint**: Summarize the entire process to see how each step leads to the final result.

### Final Answer:
The Prussian blue color obtained during the test of nitrogen by Lassaigne's test is due to the formation of iron(III) hexacyanoferrate(II), represented by the formula Fe₄[Fe(CN)₆]₃.