(d) Passivity of iron is due to the formation of thin film of ____on its surface.

Explanation

**Step-by-Step Text Solution:**

1. **Understanding Passivity**:
- Passivity refers to the property of a metal to become inert or less reactive under certain conditions. This means that even when chemical activity is expected, the metal does not react as it normally would.

2. **Formation of a Protective Layer**:
- In the case of iron, when it is exposed to certain chemicals, such as concentrated nitric acid (HNO3), it can undergo a reaction that leads to the formation of a protective layer on its surface.

3. **Chemical Reaction**:
- When iron is treated with concentrated nitric acid, it reacts to form iron oxide. Specifically, the oxide that forms is Fe3O4, which is also known as magnetite.

4. **Role of the Oxide Layer**:
- This iron oxide layer (Fe3O4) acts as a barrier that protects the underlying iron from further corrosion and chemical attack. This is why iron exhibits passivity in the presence of certain reagents; the oxide layer prevents direct contact between the iron and the corrosive substances.

5. **Conclusion**:
- Therefore, the passivity of iron is due to the formation of a thin film of iron oxide (Fe3O4) on its surface.

**Final Answer**: The passivity of iron is due to the formation of a thin film of **iron oxide (Fe3O4)** on its surface.

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