D. is not chemical weathering.
We know this because when iron oxides, it is reacting with oxygen in the air, with is therefore a chemical change.
When clay forms, it's from the chemical deposition of rocks like granite, which makes it a type of chemical weathering.
Finally, limestone reacting with acid rain is a chemical change, which makes it chemical weathering.
A rock being broken down does not change the chemical structure of the rock, so it is a physical change, and physical weathering, not chemical.