Answer:
The height of the plants
Step-by-step explanation:
In an experimental design, there are two types of variables: independent and dependent variables. The dependent variable is modified by the independent one. In other words, researchers can change the independent variable to see the possible effect in the dependent one. In this example, the study wants to see the effect of the amount of water (independent variable) on plant growth (dependent variable). They evaluate the height of the plants (growth) exposed to different volumes of water. So, the variable of interest in this study is the height of the plants.