Ferric ion forms a Prussian blue precipitate due to the formation of

The reagent used for the detection of $Ni^{2+}$ is $A$. $x$ moles of $A$ react with $Ni^{2+}$ to form a complex of $C$ color. $A$,$x$,and $C$ are respectively:

If a didentate ligand ethane-$1, 2$-diamine is progressively added in the molar ratio $en : Ni :: 1: 1, 2: 1, 3: 1$ to $[Ni(H_2O)_6]^{2+}$ aqueous solution,the following coordination entities are formed:
$I$. $[Ni(H_2O)_4en]^{2+}_{(aq)} -$ pale blue
$II$. $[Ni(H_2O)_2(en)_2]^{2+}_{(aq)} -$ blue/purple
$III$. $[Ni(en)_3]^{2+}_{(aq)} -$ violet
The wavelengths in $nm$ of light absorbed in the case of $I$ and $III$ are respectively:

Assign $A, B, C, D$ from the given type of reaction.
$2K_3[Fe(CN)_6] + 3FeCl_2 \longrightarrow Fe_3[Fe(CN)_6]_2 \downarrow + 6KCl$

Assign $A, B, C, D$ from the given type of reaction.
$NiCl_2 + 2dmg \xrightarrow{NH_4OH} Ni(dmg)_2 \downarrow$

Sodium nitroprusside when added to an alkaline solution of sulphide ions produces a: