1. Identify the reaction involved

Acidic ferric chloride solution contains $Fe^{3+}$ ions. On adding excess potassium ferrocyanide, $K_4[Fe(CN)_6]$, the ferrocyanide ion is $[Fe(CN)_6]^{4-}$.

The well-known reaction is:

$Fe^{3+} + [Fe(CN)_6]^{4-} \longrightarrow \text{Prussian blue}$

2. What is Prussian blue?

Prussian blue is the deep blue ferric ferrocyanide formed when $Fe^{3+}$ reacts with ferrocyanide ion.

Its classical composition is:

$Fe_4[Fe(CN)_6]_3$

Let us check charge balance:

• $4$ ferric ions contribute: $4 \times (+3) = +12$
• $3$ ferrocyanide ions contribute: $3 \times (-4) = -12$

So the compound is electrically neutral:

$Fe_4[Fe(CN)_6]_3$

3. Evaluate the options

• A: $Fe_4[Fe(CN)_6]_3$
  This is ferric ferrocyanide, i.e. Prussian blue. Correct.

• B: $K_5Fe[Fe(CN)_6]_2$
  This is not the standard formula for Prussian blue obtained in this test.

• C: $HFe[Fe(CN)_6]$
  Not the known composition of Prussian blue.

• D: $KFe[Fe(CN)_6]$
  Also not Prussian blue.

4. Final answer

Therefore, the Prussian blue coloured colloidal species is:

$\boxed{Fe_4[Fe(CN)_6]_3}$

So, Option A is correct.