What condition is required to rust in iron?

presence of O₂

The correct answer is option 4. presence of O_2.

Rusting, or corrosion of iron, is a chemical reaction where iron reacts with oxygen and water to form iron oxide, commonly known as rust. This process occurs in the presence of oxygen \((O_2)\) and water \((H_2O). The reaction can be represented as:

\(Fe + O_2 + H_2O \longrightarrow \underset{\text{Iron oxide (Rust)}}{Fe_2O_3\, \ or\, \ Fe_3O_4}\)

The presence of oxygen is crucial for rusting to occur because iron atoms need to react with oxygen atoms to form iron oxide. Without oxygen, the rusting process cannot take place.

Therefore, the correct condition required for rusting in iron is the presence of oxygen \((O_2)\).