1. Lassaigne test for nitrogen

   In Lassaigne's test, an organic compound containing nitrogen is fused with sodium. Nitrogen gets converted into sodium cyanide:

   $\text{Na} + \text{C} + \text{N} \rightarrow \text{NaCN}$

2. Reaction with ferrous sulphate

   The Lassaigne extract containing $\text{CN}^-$ is treated with freshly prepared $FeSO_4$. This forms sodium ferrocyanide:

   $6NaCN + FeSO_4 \rightarrow Na_4[Fe(CN)_6] + Na_2SO_4$

3. Oxidation and acidification

   On acidification and in presence of ferric ions, sodium ferrocyanide gives ferric ferrocyanide, which is the characteristic Prussian blue precipitate:

   $3Na_4[Fe(CN)_6] + 4Fe^{3+} \rightarrow Fe_4[Fe(CN)_6]_3 + 12Na^+$

4. Identification of the compound formed

   The blue coloured compound formed in the positive nitrogen test is:

   $Fe_4[Fe(CN)_6]_3$

5. Check options

   • A: $Fe_4[Fe(CN)_6]_3$ — correct, Prussian blue.
   • B: $Na_4[Fe(CN)_5NOS]$ — this is related to the test for both nitrogen and sulphur together (sodium nitroprusside type species), not the nitrogen-only test.
   • C: $Fe(CN)_3$ — not the characteristic product.
   • D: $Na_3[Fe(CN)_6]$ — ferricyanide, not the final blue precipitate in the nitrogen test.

6. Final answer

   The compound formed in the positive test for nitrogen is:

   $\boxed{Fe_4[Fe(CN)_6]_3}$

This matches the stored correct answer.