The right option is; C. Amount of fertilizer
Amount of fertilizer is the factor that is being tested in the experiment.
In this experiment, different amounts of fertilizer that was mixed with distilled water was added to three of the containers. The different amount of the applied fertilizer is expected to have different effect and to produce different results. This makes the amount of fertilizer to be the factor that is being tested in the experiment. The container to which only distilled water is added will serve as the control in the experiment.