Extraction of $Ag$ from sulphide ore and removal of unreacted silver from photographic plate involve complexes are respectively

Coordination compounds are of great importance in biological systems. In this context,which of the following statements is incorrect?

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:

$A$ solution of $FeCl_3$ when treated with $K_4[Fe(CN)_6]$ gives a Prussian blue precipitate due to the formation of:

Schweitzer's reagent,used for dissolving cellulose in the manufacture of artificial silk,is

Sodium thiosulphate is used in photography for which of the following purposes?