Which of the following is necessary requirement for a piece of iron to get rusted?

Both Oxygen and water vapour

The correct answer is option 3. Both Oxygen and water vapor.

The process of rusting can be represented by the following equation:

Iron (Fe) + Oxygen (O₂, from the air) + water (H₂O) → Rust (iron oxide Fe₂O₃)

For rusting, the presence of both oxygen and water (or water vapour) is essential.

In fact, if the content of moisture in air is high, which means if it is more humid, rusting becomes faster.