Final answer:
In the described experimental study, the most appropriate control group is areas with historically stable rainfall patterns (Option B), the independent variable is El Niño intensity, and the dependent variable is the amount of local rainfall measured.
Step-by-step explanation:
In an experimental study, identifying the control group, independent variable, and dependent variable is crucial for understanding the effects of the experiment. The control group serves as a benchmark to compare the experimental group against, and they should be as similar as possible except for not receiving the experimental manipulation. In this context, the independent variable, which is the factor being manipulated by researchers, is the intensity of El Niño, measured by sea surface temperatures off the coast of Ecuador. Conversely, the dependent variable is what researchers measure in response to changes in the independent variable; here, it is the amount of rainfall in the southeastern United States, quantified by rain gauges.
Among the given options, the most suitable control group would be one that is unaffected by El Niño. While Options A and D suggest control groups that are either without the influence of El Niño or during a previous El Niño event, they may not be as stable or as unaffected as Option B, which describes areas with historically stable rainfall patterns. Option C's randomness does not guarantee stability or a lack of El Niño influence. Therefore, Option B seems to provide the most stable benchmark for comparison, with historically stable rainfall patterns serving as the control group.