Final answer:
The choice that is not a sign of a chemical reaction occurring is D. Change in size.
Step-by-step explanation:
The correct choice that is not a sign of a chemical reaction occurring is D. Change in size.
Chemical reactions are characterized by certain observable changes, such as temperature change (A), gas produced (B), and unexpected color change (C), as mentioned in the other choices. However, a change in size (D) is not typically a reliable indicator of a chemical reaction.
For example, cutting a piece of paper into smaller pieces does not involve a chemical reaction, but a physical change in size. Likewise, a physical change in size can occur when stretching a rubber band. Therefore, choice D is not a sign of a chemical reaction occurring.
Learn more about Signs of chemical reactions