Final answer:
The independent variable in Gabriel's experiment with tomato plants is the act of speaking to the plants, chosen to evaluate its effect on plant height while controlling other factors.
Step-by-step explanation:
In Gabriel's investigation of tomato plants, the independent variable is the act of speaking to the plants.
The independent variable is the one that is changed or manipulated by the experimenter to observe its effect on the dependent variable, which in this case is the height of the tomato plant.
The rationale behind choosing speaking to the plants as the independent variable is that it represents a factor that can be systematically varied to determine if it has any impact on plant growth, while all other variables remain constant.