Final answer:
Dr. Mason's study is an experimental study due to the intentional manipulation of the caffeine levels to measure their effect on word recall, which allows for observation of cause-and-effect.
Step-by-step explanation:
Dr. Mason conducted an experimental study. In this study, he manipulated the amount of caffeine, which is the independent variable, to see its effect on the participants' word recall, the dependent variable. The hallmark of an experimental study is this active manipulation along with the control of other variables to isolate the effect of the independent variable on the dependent variable.
In contrast to observational studies, where researchers simply observe and measure variables without intervention, an experimental research design allows researchers to establish cause-and-effect relationships between variables. As Dr. Mason is deliberately varying the amount of caffeine given to participants and then measuring the resultant impact on their word recall, he is not merely observing the participants but actively changing a condition to observe the outcome.