Which of the following is an example of a research method requiring control over variables?

The correct choice is an experiment, as this research method is specifically designed to investigate causal relationships by controlling for variables. In an experiment, researchers manipulate one or more independent variables to observe their effect on dependent variables, while keeping all other potential confounding variables constant. This control is essential for establishing a cause-and-effect relationship, as it minimizes the influence of extraneous factors that could distort the findings.

Surveys, while valuable for gathering data and insights, do not manipulate variables and often rely on self-reported information without the structured control seen in experiments. Similarly, case-control studies and observational studies focus on existing conditions or characteristics rather than actively controlling or manipulating variables. In these latter methods, researchers may identify associations or correlations but cannot definitively conclude causal relationships due to the lack of variable control.