What stays constant in a reaction? a. the total volume b. the amount of liquid c. the amount of gas d. the total mass

Correct answers is D.

Explanation:
Reaction most often happens between two chemicals. But it can also happen with only one chemical. Heating of water is also a reaction.

When we heat the water we bring thermal energy to it. This causes raise of temperature and water starts boiling. This creates water vapour. In this way the amount of water in liquid and gas states changes. This eliminates answers B and C.

Freezing of water is also a reaction. When the water freezes it increases it's volume. This eliminates answer A.