Prussian blue is -

Options

A. `Na_(4)Fe(CN)_(6)`
B. `Na_(3)Fe(CN)_(6)`
C. `Fe_(4)[Fe(CN)_(6)]_(3)`
D. None of these

Answer

To determine what Prussian blue is, we need to analyze the options given and understand the chemistry behind Prussian blue formation.

### Step-by-Step Solution:

1. **Understanding Prussian Blue**: Prussian blue is a complex compound formed from iron and cyanide ions. It is known for its deep blue color.

2. **Identifying the Chemical Formula**: The chemical formula for Prussian blue can be represented as \( Fe_4[Fe(CN)_6]_3 \). This indicates that it consists of iron ions and cyanide ions.

3. **Analyzing the Options**:
- **Option 1**: \( Na_4[Fe(CN)_6] \) - This is not Prussian blue; it is a different complex known as sodium ferrocyanide.
- **Option 2**: \( Na_3[Fe(CN)_6] \) - This is also not Prussian blue; it is sodium ferricyanide.
- **Option 3**: \( Fe_4[Fe(CN)_6]_3 \) - This matches the known formula for Prussian blue.
- **Option 4**: \( Na_2SO_4 \) - This is sodium sulfate and not related to Prussian blue.

4. **Conclusion**: The correct answer is **Option 3: \( Fe_4[Fe(CN)_6]_3 \)**, which is the formula for Prussian blue.