Explanation:

- Weathering (Option 3) : This is the correct answer. It involves the breaking down of rocks due to various environmental factors like temperature changes, frost, plants, animals, and human activity.

- Smelting (Option 1): This is a process used to extract metal from ore, not related to breaking down rocks.

- Erosion (Option 2): This refers to the movement of rock particles by wind, water, or ice. It typically occurs after weathering.

- Deforestation (Option 4): This involves the removal of forests and does not directly relate to the breaking up of rocks.