Which of the following is not an example of chemical weathering?
a. the oxidation of iron
b. the formation of clay
c. the reaction of limestone with acid rain
d. the breaking of a rock into smaller pieces

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.

Thermohaline circulation and gyres

Hi + naoh -> h2o + nal