Final answer:
The independent variable in this study is the fertilizer type, while the dependent variable is plant growth.
Step-by-step explanation:
The independent variable in this study is c) Fertilizer type. In an experiment, the independent variable is the factor that the researcher deliberately manipulates or changes to observe its effect on the dependent variable. In this case, the farmer is testing different fertilizer types to see how they affect plant growth. The dependent variable is the factor that is being measured or observed, which in this study is a) Plant growth.