If you were testing how well different dish soap cleaned grease, the independent (experimental) variable would be the:

The experimental testing done in accordance to the experimental research method of hypothesis testing would conclude that the independent variable be dish soap.

Explanation:

The experiment for the quality of grease cleaning dish soap, the most dependent factor is the dishes and the types of greases on it. The independent variable thus be the objective variable of the experiment which is the dish soap.

Dish soap is to be tested for the cleaning of grease and thus the dishes and water and the type of greases remains constant or similar for the experiment.