Answer: A. chemical weathering by oxidation
Weathering is a natural process in which the rock is broken down into small pieces and sediments under the influence of agents like wind, water and chemicals. Chemical weathering is a process in which the rocks breaks down due to the effect of the chemicals. It brings change in the chemical composition of the broken pieces and sediments distinct from the parent rock.
Rust on rocks is a an example of chemical weathering by oxidation. The rock mainly exhibiting elements which can be easily oxidized such as iron undergoes rusting this occurs because oxygen breaks the bonds between the elements in the rock. The internal structure of the rock becomes weak and results in rusting.