c
\((c)\) In Lassaigne's test, fusion with sodium take place and following species formed respectively.

\((a)\) Aniline \(\to \) \(CN^-\)

\((b)\) Benzene sulfonic acid \(\to \) (figure)

\((c)\) Thiourea \(\to \) \(S^{2-}\)

Reaction of \(CN^-\) with hot and acidic solution of \(FeSO_4\) lead to formation of \(Fe_4[Fe(CN)_6]_3\) which is blue in colour. It contains iron in both \(II\) and \(III\) oxidation state.

Reaction of \(S^{2-}\) with sodium nitroprusside

\(N{a_2}S + N{a_2}[Fe{(CN)_5}NO] \to \mathop {N{a_4}[Fe{{(CN)}_5}NOS]}\limits_{(violet\,in\,colour)} \)

Phenoxide ion on reacting with \(FeCl_3\) give red colour with \(FeCl_3\).