Answer: C. The amount of stain on the cloth.
Step-by-step explanation:
In an experimental procedure the cause and effect relationships can be determined by assigning two variables that are dependent and independent variables. The independent variable is the variable which exerts it's influence over the changes occurring on the dependent variable. The dependent variable helps in generating the result of the experiment.
The soap is the independent variable the effect of this can be observed on the amount of stain left after scrubbing the piece of grass-stained cloth. Hence, the amount of stain left on the cloth is the dependent variable.