Final answer:
The independent variable in the study is the types of grasses, as this would be the factor manipulated by the researcher to observe its effects on soil erosion.
Step-by-step explanation:
The independent variable or manipulated variable is the factor that is changed by the researcher to observe how it affects the dependent variable. In the context of the question provided, the independent variable is option A. The types of grasses. This is because it is the factor that would be changed to see how it affects soil erosion, which would be the dependent variable - the outcome measured in the study. Factors like the size of plots, type of soil, or the amount of soil erosion are not manipulated, making them either dependent variables or control variables.