Lassaigne's test is used to detect the presence of nitrogen and sulfur in organic compounds. The test involves fusing the organic compound with sodium metal, which forms sodium cyanide and sodium sulfide. These products are then dissolved in water and tested for the presence of nitrogen and sulfur separately.

Formation of NaCN and Na2S
When an organic compound containing nitrogen and/or sulfur is fused with sodium metal, the following reactions take place:

1. Nitrogen:
Organic nitrogen compounds react with sodium metal to form sodium cyanide (NaCN) according to the following reaction:
R-NH2 + Na → R-Na+ + Na+ + 1/2H2

2. Sulfur:
Organic sulfur compounds react with sodium metal to form sodium sulfide (Na2S) according to the following reaction:
R-SH + 2Na → R-Na+ + Na+ + H2S

Separation of NaCN and Na2S
After the fusion reaction, the products (NaCN and Na2S) are dissolved in water and the solution is divided into two parts for separate tests for nitrogen and sulfur.

Test for Nitrogen
The solution containing NaCN is acidified with dilute H2SO4 and then treated with FeSO4 solution. The presence of nitrogen is indicated by the formation of Prussian blue color.

Test for Sulfur
The solution containing Na2S is acidified with dilute HCl and then treated with lead acetate solution. The presence of sulfur is indicated by the formation of black lead sulfide (PbS) precipitate.

Conclusion
In conclusion, during Lassaigne's test, nitrogen and sulfur present in an organic compound change into NaCN and Na2S respectively. The products are then dissolved in water and tested separately for the presence of nitrogen and sulfur using different reagents.

We prepare sodium fusion extract i.e, lassaignes extract to conver element into ionic compounds,
organic+Na metal 》powder + H2O 》NaCN,P,Nax,Na2S
if N,S are present , NaSCN +Na excess gives rise to Na2S +NaCN so option b is correct