Which color of light is absorbed by an aqueous solution of $CuSO_4$?

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

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:

Discuss briefly,giving an example in each case,the role of coordination compounds in:
$(i)$ biological systems
$(ii)$ medicinal chemistry
$(iii)$ analytical chemistry
$(iv)$ extraction/metallurgy of metals

Which among the following is used in the treatment of cancer?

$A$ reagent which gives a brilliant red precipitate with $Ni^{2+}$ ions in a basic medium is