Iron is rendered passive by treatment with

Options

A. `H_2SO_4` is a strong oxidising agent and it reacts with `KMnO_4` during titration
B. `H_3PO_4`
C. `HCl`
D. conc. `HNO_3`

Explanation

To answer the question "Iron is rendered passive by treatment with," we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Passivation**:
- Passivation is a process where a metal becomes less reactive due to the formation of a protective oxide layer on its surface. This layer prevents further oxidation of the underlying metal.

2. **Analyzing the Options**:
- We have four options:
1. H2SO4 (sulfuric acid)
2. H3PO4 (phosphoric acid)
3. HCl (hydrochloric acid)
4. Concentrated HNO3 (nitric acid)

3. **Evaluating Each Option**:
- **H2SO4**: While sulfuric acid is a strong oxidizing agent, it does not passivate iron effectively. It can react with iron but does not form a protective oxide layer.
- **H3PO4**: Phosphoric acid does not passivate iron. It can react with iron but does not create a protective layer.
- **HCl**: Hydrochloric acid reacts with iron, dissolving it, but does not lead to passivation.
- **Concentrated HNO3**: Concentrated nitric acid is known to passivate iron. When iron is treated with concentrated nitric acid, it forms a protective oxide layer that prevents further corrosion.

4. **Conclusion**:
- Based on the evaluation, the correct answer is that iron is rendered passive by treatment with **concentrated HNO3 (nitric acid)**.

### Final Answer:
Iron is rendered passive by treatment with **concentrated HNO3 (nitric acid)**.

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