Which one among the following is an example of oxidation reaction ?

An oxidation reaction is a chemical process involving the loss of electrons or the addition of oxygen to a substance. Rusting of iron is a classic example of this process.

When iron is exposed to oxygen and moisture (water) over time, it undergoes a redox reaction to form hydrated iron(III) oxide ($Fe_2O_3 \cdot nH_2O$), commonly known as rust. In this reaction, iron atoms lose electrons (oxidation) to oxygen molecules.

The other options are categorized as physical changes, not chemical reactions:

• Freezing of water and Boiling of petrol are phase transitions (liquid to solid and liquid to gas, respectively) where the chemical identity remains unchanged.
• Dissolving sugar in water is a physical process of solvation where sugar molecules disperse in water without forming new chemical substances.