Which process is a form of mechanical weathering?
a. hydration
b. carbonation
c. exfoliation
d. oxidation

Mechanical weathering is actually breaking down of rocks into smaller pieces by natural forces. Any weathering processes that can cause the physical breakdown of rocks without any type of change in the chemical composition of rocks, called mechanical weathering.

So, looking at each of the definitions in the options, we can easily choose:
1. Hydration : It's a process of absorbing water by substance.

2. Carbonation: It's a process of Carbon Dioxide dissolving in liquid (mostly water).

3. Oxidation : It's a process of oxygen reacting with some element.

4. Exfoliation:  It's a process where the rocks erodes by peeling off in sheets or layer by layer rather than grain by grain.

As you can see the last one Exfoliation matches with the definition of mechanical weathering. It's one of its types.