Aluminium becomes passive in :

Options

A. conc `HNO_(3)`
B. `H_(2)CrO_(4)`
C. `HClO_(4)`
D. conc `HCl`

Explanation

To determine when aluminum becomes passive, we can follow these steps:

### Step-by-Step Solution:

1. **Understanding Passivation**:
- Passivation refers to the process where a metal becomes less reactive or inert due to the formation of a protective layer on its surface.

2. **Identifying the Agent**:
- In this case, we need to identify the conditions under which aluminum becomes passive. One well-known agent that can cause passivation in aluminum is concentrated nitric acid (HNO3).

3. **Reaction with Concentrated HNO3**:
- When aluminum is treated with concentrated nitric acid, a chemical reaction occurs. The aluminum reacts with nitric acid to form aluminum oxide (Al2O3).

4. **Formation of Protective Coating**:
- The aluminum oxide (Al2O3) that forms acts as a protective coating on the surface of the aluminum. This layer prevents further oxidation and protects the underlying metal from reacting with other substances.

5. **Oxidation-Reduction Process**:
- During this reaction, aluminum acts as a reducing agent and oxidizes itself to form aluminum oxide. This is a key aspect of the passivation process.

6. **Conclusion**:
- Therefore, the correct answer to the question "Aluminium becomes passive in:" is when it reacts with concentrated HNO3, leading to the formation of Al2O3.

### Final Answer:
Aluminum becomes passive when treated with concentrated HNO3.

---