Final answer:
The subject of this question is Chemistry, and the correct answer is chemical reactions.
Step-by-step explanation:
The subject of this question is Chemistry.
In the given options, the correct answer is b) Chemical reactions. In Chemistry, chemical reactions are the focus of study, where substances undergo a change in their chemical composition through the breaking and formation of bonds. These reactions can be observed through various signs, such as color change, gas production, or temperature change.