Answer:Number of plants.
Step-by-step explanation:
When any experiment is conducted then there are two variables in the experiment. One of the variable is independent variable that we can change according to the need of the experiment.
Then the result of the experiment is known as dependent variable which depends on the independent variable.
Here, the concentration of carbon dioxide can be changed and is known as independent variable but the number of plants that will grow in the given carbon dioxide concentrations is dependent variable.