Correct Option: B  [ increases ]
Remark:
When iron rusts, its weight increases. This is because rusting is a chemical reaction between iron, oxygen, and moisture from the air, forming hydrated iron(III) oxide (Fe₂O₃·xH₂O). During this process, oxygen atoms from the air combine with the iron, adding extra mass to the metal surface in the form of rust. However, while the weight increases, the structural strength of the iron decreases due to corrosion.
Other options (for context)
- Decreases: Incorrect — mass increases due to the addition of oxygen.
- Remains the same: Incorrect — oxygen from air chemically adds to iron, increasing total mass.
- First increases and then decreases: Not applicable — in normal rusting, weight continues to increase until the surface flakes off.