Recall the test involved

In Lassaigne's test for sulphur, the sodium extract contains sulphur as sodium sulphide, $Na_2S$.

When this extract is treated with sodium nitroprusside, a purple/violet colour appears.

Identify the reagent

Sodium nitroprusside has the formula: $Na_2[Fe(CN)_5(NO)]$

This is the reagent used in the test, but it is not the purple-coloured product.

Reaction with sulphide ion

The sulphide ion, $S^{2-}$, reacts with nitroprusside ion: $[Fe(CN)_5(NO)]^{2-} + S^{2-} \rightarrow [Fe(CN)_5(NOS)]^{4-}$

Therefore, the coloured complex formed is: $Na_4[Fe(CN)_5(NOS)]$

Check the options

• A: $NaFe[Fe(CN)_6]$
  This is not related to the sulphur nitroprusside purple complex.

• B: $Na[Cr(NH_3)_2(NCS)_4]$
  Incorrect; unrelated complex.

• C: $Na_2[Fe(CN)_5(NO)]$
  This is sodium nitroprusside itself, the reagent, not the purple product.

• D: $Na_4[Fe(CN)_5(NOS)]$
  This is the purple/violet complex formed in the test.

Final answer

Hence, the formula of the purple colour formed is: $\boxed{Na_4[Fe(CN)_5(NOS)]}$

So, the correct option is D.