Identify the reagent

Potassium ferrocyanide is: $K_4[Fe(CN)_6]$

It contains the complex ion: $[Fe(CN)_6]^{4-}$

Recall the characteristic test

Prussian blue is a deep blue coloured compound formed when ferric ions $Fe^{3+}$ react with ferrocyanide ions $[Fe(CN)_6]^{4-}$.

The reaction is commonly represented as: $4Fe^{3+} + 3[Fe(CN)_6]^{4-} \rightarrow Fe_4[Fe(CN)_6]_3$

This compound is called Prussian blue.

Check each option

A: $CoCl_3$

• Provides $Co^{3+}$ ions, not $Fe^{3+}$.
• Does not give Prussian blue.

B: $FeCl_2$

• Provides $Fe^{2+}$ ions.
• $Fe^{2+}$ with ferrocyanide gives a pale/white precipitate (ferrous ferrocyanide), not Prussian blue.
• So this is not correct.

C: $CoCl_2$

• Provides $Co^{2+}$ ions.
• Does not give Prussian blue.

D: $FeCl_3$

• Provides $Fe^{3+}$ ions.
• $Fe^{3+}$ reacts with potassium ferrocyanide to give Prussian blue.
• Hence this is correct.

Conclusion

The solution of potassium ferrocyanide gives Prussian blue colour with: $FeCl_3$

So the correct option is D.