`"………….."` is produced by action of conc. `H_(2)SO_(4)` on potassium ferrocyanide.

Answer

To solve the question about what is produced by the action of concentrated sulfuric acid (H₂SO₄) on potassium ferrocyanide (K₄[Fe(CN)₆]), we can follow these steps:

### Step-by-Step Solution:

1. **Identify Potassium Ferrocyanide**:
- Potassium ferrocyanide is represented as K₄[Fe(CN)₆]. It contains iron in the +2 oxidation state and cyanide ions.

2. **Understand the Reaction**:
- When potassium ferrocyanide reacts with concentrated H₂SO₄, the sulfuric acid acts as a dehydrating agent and an oxidizing agent.

3. **Oxidation Process**:
- The concentrated H₂SO₄ oxidizes the ferrocyanide ion (Fe²⁺) to ferric ion (Fe³⁺). This oxidation process can lead to the formation of iron(II) sulfate (FeSO₄) and other products.

4. **Formation of Carbon Monoxide**:
- The reaction also produces carbon monoxide (CO) gas as a significant product. This occurs due to the breakdown of the cyanide ions (CN⁻) in the presence of concentrated sulfuric acid.

5. **Final Products**:
- The main products of the reaction are carbon monoxide (CO) gas and iron(II) sulfate (FeSO₄), along with ammonium sulfate (NH₄)₂SO₄.

6. **Conclusion**:
- Therefore, the answer to the question is that carbon monoxide (CO) is produced by the action of concentrated H₂SO₄ on potassium ferrocyanide.

### Final Answer:
**Carbon monoxide (CO)** is produced by the action of concentrated H₂SO₄ on potassium ferrocyanide.