Prussian blue is a dark blue pigment that is commonly used in art and as a laboratory reagent. It is formed when ferric chloride (FeCl3) reacts with potassium ferrocyanide (K4[Fe(CN)6]).

Here is a detailed explanation of why option B is the correct answer:

1. Introduction:
Prussian blue is a coordination compound with the chemical formula Fe4[Fe(CN)6]3. It is an inorganic pigment that is known for its deep blue color. The compound is formed through a chemical reaction between ferric chloride and potassium ferrocyanide.

2. Ferric chloride (FeCl3):
Ferric chloride is a compound that contains iron in its +3 oxidation state. It is a yellowish-brown solid that is highly soluble in water. In aqueous solutions, it dissociates into Fe3+ ions and chloride ions (Cl-).

3. Potassium ferrocyanide (K4[Fe(CN)6]):
Potassium ferrocyanide is a coordination compound that contains iron in its +2 oxidation state. It is a yellow crystal that is also highly soluble in water. In aqueous solutions, it dissociates into K+ ions and the complex ion [Fe(CN)6]4-.

4. Reaction between FeCl3 and K4[Fe(CN)6]:
When ferric chloride reacts with potassium ferrocyanide, the following reaction occurs:

FeCl3 + 3K4[Fe(CN)6] → Fe4[Fe(CN)6]3 + 12KCl

In this reaction, the Fe3+ ions from ferric chloride combine with the [Fe(CN)6]4- ions from potassium ferrocyanide to form the Prussian blue complex ion Fe4[Fe(CN)6]3. The reaction also produces 12 moles of potassium chloride (KCl) as a byproduct.

5. Formation of Prussian blue:
The Prussian blue complex ion Fe4[Fe(CN)6]3 is responsible for the deep blue color of Prussian blue. It consists of a central iron atom surrounded by six cyanide ligands. The coordination geometry of the complex ion is octahedral.

6. Significance of the reaction:
The reaction between ferric chloride and potassium ferrocyanide is commonly used to synthesize Prussian blue in the laboratory. It is a simple and efficient method for producing the pigment.

In conclusion, Prussian blue is formed when ferric chloride reacts with potassium ferrocyanide. This reaction results in the formation of the Prussian blue complex ion Fe4[Fe(CN)6]3, which is responsible for the deep blue color of the pigment.