(D)

If the salt contains sulphide or cyanide, then Lassaigne’s extract would contain ${\mathrm{Na}}_2\mathrm{S}$ or NaCN. In such cases, upon the addition of ${\mathrm{AgNO}}_3$ in the extract, ${\mathrm{Ag}}_2\mathrm{S}$ (black precipitate) or AgCN (white residue) are obtained which interferes with the test for halides. Hence Lassaigne’s extract is boiled with dil. ${\mathrm{HNO}}_3$ to decompose ${\mathrm{Na}}_2\mathrm{S}$ or NaCN.

(C)

Nitrogen, sulphur, halogens and phosphorus present in an organic compound are detected by “Lassaigne’s test”.

(B)

In Lassaigne's test for nitrogen, NaCN is formed in sodium fusion extract, which is then tested further. Hydrazine (${\mathrm{NH}}_2-{\mathrm{NH}}_2$) does not contain carbon hence NaCN cannot be formed in sodium fusion extract. So hydrazine does not give Lassaigne 's test for nitrogen.