Answer:
The amount (number) of mycorrhizal fungi
Explanation:
In a scientific experiment/study, the independent variable is the variable that isn't changed by other variables, while the dependent variable is the variable that is being measured during the experimental procedure. The independent variable is manipulated by the researcher (in this case, the amount of mycorrhizal fungi), thereby affecting the dependent variable (in this case, grass growth). In consequence, a variable is dependent because it 'depends' on the independent variable.