Final answer:
The best choice for a dependent variable for Bill, who wants to use a changing criterion design, is the duration of time spent studying as it directly reflects the study behavior he wishes to change. The correct answer is option: d. Duration of time spent studying.
Step-by-step explanation:
The dependent variable in research is the variable that is being affected or measured to see how it is influenced by the independent variable. In the case of Bill wanting to use a changing criterion design to help him study more, the best choice for a dependent variable would be the duration of time spent studying. This is because the primary goal is to increase study time, so the effect of the changing criteria (independent variable) on the study time (dependent variable) should be observed.
Other options such as the impact of activities completed, frequency of breaks, or latency of interventions implemented do not directly measure the amount of time studying, which is the focus of Bill's concern.
Choosing the correct dependent variable is essential for the success of an experimental study as it directly determines if the changes in the independent variable cause a variation in the outcome being observed.