The metal cation present in sodium nitroprusside is also present in:

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 the correct code for Column-$I$ and Column-$II$:

Column-$I$ (Coordination compound)	Column-$II$ (Central metal atom)
$(a)$ Chlorophyll	$(p)$ Rhodium
$(b)$ Blood pigment	$(q)$ Cobalt
$(c)$ Wilkinson catalyst	$(r)$ Calcium
$(d)$ Vitamin $B_{12}$	$(s)$ Iron
	$(t)$ Magnesium

When $FeCl_3$ reacts with $K_4[Fe(CN)_6]$,which of the following compounds is formed?

Identify the complexes formed in the following processes:
$I$. Mond's process for purification of nickel
$II$. Removal of unreacted $AgBr$ from photographic plates
$III$. Removal of lead poisoning from the body

In photography,sodium thiosulphate is used as: