Identify the common names / known colours of the given iron complexes

We use standard qualitative inorganic chemistry facts:

• $[Fe(CN)_6]^{4-}$ is ferrocyanide.
• $[Fe(SCN)_6]^{4-}$ is a thiocyanato iron complex.
• $Fe_4[Fe(CN)_6]_3\cdot H_2O$ is Prussian blue (or a closely related ferric ferrocyanide complex), known for its deep blue colour.
• $[Fe(CN)_5NOS]^{4-}$ is related to sodium nitroprusside test species, known to give a violet/purple colour.

Evaluate each option

Option A: $[Fe(CN)_6]^{4-}$

This is ferrocyanide ion.

Its salts are generally pale yellow.

So, this is not violet.

Option B: $[Fe(SCN)_6]^{4-}$

Iron-thiocyanate complexes are generally associated with blood-red / red coloration.

So, this is not violet.

Option C: $Fe_4[Fe(CN)_6]_3\cdot H_2O$

This is Prussian blue.

As the name suggests, it is blue, not violet.

So, this is not violet.

Option D: $[Fe(CN)_5NOS]^{4-}$

This corresponds to the coloured species formed in the nitroprusside test, which is known for a violet/purple colour.

Hence, this is the violet-coloured complex.

Final answer

Therefore, the correct option is: $\boxed{D}$

Comparison with stored correct answer

Stored correct answer: $D$

My derived answer: $D$

They match.