b
If the organic compound also contains nitrogen or sulphur, the Lassaigne's extract on boiling with dil. \(HNO_3\) decomposes sodium cyanide or sodium sulphide formed during fusion.

 \(NaCN+HN{{O}_{3}}\to NaN{{O}_{3}}+HCN\uparrow \)

\(N{{a}_{2}}S+2HN{{O}_{3}}\to 2NaN{{O}_{3}}+{{H}_{2}}S\uparrow \)

If cyanide and sulphide ions are not decomposed, they will react with silver nitrate and hence will interfere with the test.

\(NaCN+AgN{{O}_{3}}\to \underset{\begin{smallmatrix} 
 Silver \\ 
 cyanide 
 \\ 
 (White\,ppt.) 
\end{smallmatrix}}{\mathop{AgCN}}\,+NaN{{O}_{3}}\)

\(N{{a}_{2}}S+2AgN{{O}_{3}}\to \underset{\begin{smallmatrix} 
 Silver \\ 
 sulphide 
 \\ 
 (Black\,ppt.) 
\end{smallmatrix}}{\mathop{A{{g}_{2}}S}}\,+2NaN{{O}_{3}}\)