Chocolate brown precipitate is formed with:

Options

A. `Cu^(2+)` ions and `[Fe (CN)_(6)]^(3-)`
B. `Cu^(2+)` ions and `[Fe (CN)_(6)]^(4-)`
C. `Fe^(2+)` ions and `[Fe (CN)_(6)]^(4-)`
D. `Fe^(2+)` ions and dimethylglyoxime

Explanation

To determine which compound forms a chocolate brown precipitate, we will analyze the provided options step by step.

### Step-by-Step Solution:

1. **Identify the Options:**
- Option A: Cu²⁺ ions and Fe(CN)₆³⁻
- Option B: Cu²⁺ ions and Fe(CN)₆⁴⁻
- Option C: Fe²⁺ ions and Fe(CN)₆⁴⁻
- Option D: Fe²⁺ ions and dimethylglyoxime

2. **Analyze Option A:**
- When Cu²⁺ ions react with Fe(CN)₆³⁻, the product formed is Cu₃[Fe(CN)₆]₂.
- The precipitate formed is green in color.
- **Conclusion:** Option A does not produce a chocolate brown precipitate.

3. **Analyze Option B:**
- When Cu²⁺ ions react with Fe(CN)₆⁴⁻, the product formed is Cu₂[Fe(CN)₆].
- The precipitate formed is chocolate brown in color.
- **Conclusion:** Option B produces a chocolate brown precipitate.

4. **Analyze Option C:**
- When Fe²⁺ ions react with Fe(CN)₆⁴⁻, the product formed is Fe₃[Fe(CN)₆]₂.
- The precipitate formed is blue in color.
- **Conclusion:** Option C does not produce a chocolate brown precipitate.

5. **Analyze Option D:**
- When Fe²⁺ ions react with dimethylglyoxime, the product formed is a red precipitate.
- **Conclusion:** Option D does not produce a chocolate brown precipitate.

### Final Conclusion:
The only option that produces a chocolate brown precipitate is **Option B: Cu²⁺ ions and Fe(CN)₆⁴⁻**.

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