$(a)$ 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 $FeCl_3$.