The correct answer is option 3, Hydration.

Mechanical weathering refers to the physical breakdown of rocks into smaller fragments without changing their chemical composition. Chemical weathering, on the other hand, involves the chemical alteration of rocks through processes such as dissolution or hydration.

Crystallization (option 1) is a process of weathering that belongs solely to mechanical weathering. It occurs when water seeps into cracks in rocks, freezes, and expands, causing the rocks to break apart.

Exfoliation (option 2) is also a process of mechanical weathering. It occurs when layers of rock peel or flake off due to the release of pressure, such as when overlying rocks are eroded.

Hydration (option 3) is the correct answer because it involves both mechanical and chemical weathering. Hydration occurs when water combines with minerals in the rock, causing them to swell and eventually break apart.

Carbonation (option 4) is a process of chemical weathering that involves the dissolution of carbonate minerals in rocks by carbonic acid.

In summary, hydration is the correct answer as it encompasses both mechanical and chemical processes in weathering.