Iron is rendered passive by the action of

Options

A. Conc. `H_(2)SO_(4)`
B. Conc. `H_(3)PO_(4)`
C. Conc. `HCl`
D. Conc. `HNO_(3)`

Explanation

To determine which substance renders iron passive, we need to understand the concept of passivation and the role of different chemicals in this process.

### Step-by-Step Solution:

1. **Understanding Passivation**:
- Passivation is the process by which a metal becomes less reactive due to the formation of a protective oxide layer on its surface. This layer prevents further corrosion and reaction with other substances.

2. **Identifying the Chemicals**:
- The question lists several chemicals: H2H4 (Hydrazine), H3PO4 (Phosphoric acid), HCl (Hydrochloric acid), and HNO3 (Nitric acid).

3. **Analyzing Each Chemical**:
- **H2H4 (Hydrazine)**: Not typically associated with passivation of iron.
- **H3PO4 (Phosphoric acid)**: Can form a protective layer but is not the most effective for iron passivation.
- **HCl (Hydrochloric acid)**: This is a strong acid that can actually corrode iron rather than passivate it.
- **HNO3 (Nitric acid)**: This is known to oxidize iron and form a protective oxide layer (Fe2O3) on its surface, which effectively passivates the iron.

4. **Conclusion**:
- Among the given options, **HNO3 (Nitric acid)** is the correct answer as it renders iron passive by forming a protective oxide layer.

### Final Answer:
Iron is rendered passive by the action of **HNO3 (Nitric acid)**.