In organic compounds, nitrogen is tested in Lassaigne's test as 
A \[\text{F}{{\text{e}}_{\text{4}}}{{\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]}_{\text{3}}}\]
B NaCN
C \[\text{FeN}{{\text{O}}_{\text{2}}}\]​
D \[\text{NaN}{{\text{O}}_{\text{3}}}\]

Hint
The Lassaigne test is used to identify halogens, nitrogen, and sulphur in an organic molecule. The creation of a substance called Prussian blue is what gives things their blue colour.
Complete solution
For the purpose of identifying the presence of nitrogen, sulphur, and other halogens in an organic compound, we use Lassaigne's test or the sodium fusion test.
We are aware that a covalent connection holds the halogens, nitrogen, and sulphur to the organic complex. We must change the chemicals into their ionic forms in order to be able to detect their existence.
We fuse these organic molecules with sodium metal to produce their ionic versions. The fusing can be done using a variety of techniques. But typically, we use sodium metal to heat the organic chemical vigorously before mixing the heated combination with water. The ionic chemicals created during the fusion are present in the aqueous solution, and we can identify them using a variety of chemical assays.
The halogens, nitrogen, and sulphur are transformed into sodium nitride, sodium sulphide, and sodium halide, respectively, which are inorganic sodium salts.
In contrast, the fusion mixture with ferrous sulphate is boiled before strong sulphuric acid is added to it in Lassaigne's test for the detection of nitrogen. The reaction that leads to the production of the blue complex can be expressed as:
\[2NaCN+Fe{{(OH)}_{2}}\to N{{a}_{4}}Fe{{(CN)}_{6}}+NaOH\]
\[3N{{a}_{4}}Fe{{(CN)}_{6}}+4FeC{{l}_{3}}\to F{{e}_{4}}{{\left[ Fe{{(CN)}_{6}} \right]}_{3}}+NaCl\]
The appearance of blue colour is due to \[\text{F}{{\text{e}}_{\text{4}}}{{\left[ \text{Fe}{{\left( \text{CN} \right)}_{\text{6}}} \right]}_{\text{3}}}\]which is ferric ferrocyanide.

So, the correct answer is “Option A”

Note Prussian blue is the common name for the substance with the blue colour. It is a polymeric chemical that is created when ferrous ferricyanide salts are oxidised. It was the first artificial colour of the modern era. The intervalence charge transfer, which is an electron transfer between two metal sites that only differ in their oxidation states, is what gives Prussian blue its blue colour. Here, the bridging ligand CN's orbital mediates the transfer of electrons from $\pi $ orbital, producing a colour transfer band in the visible region spectra that gives the precipitate its coloured appearance.