What research method involves manipulating one or more factors to observe effects on behavior or mental processes?

The correct answer is based on the fundamental principles of experimental research methods in psychology. An experiment involves the manipulation of one or more independent variables to observe their effects on a dependent variable. This method allows researchers to establish cause-and-effect relationships by controlling extraneous variables and maintaining a systematic approach to data collection.

In an experimental setup, participants are typically assigned to different groups to compare outcomes, which enhances the reliability of the results. The ability to control conditions and variables is what distinguishes experiments from other research methods, making it possible to draw more definitive conclusions about the link between the manipulated factors and the observed behaviors or mental processes.

In contrast, surveys gather self-reported data from individuals without manipulation, correlation studies examine the relationships between variables without determining causation, and case studies provide in-depth insights into a single subject or group but do not involve manipulation of variables. Thus, the experimental method stands out as the approach specifically designed to explore the effects of manipulated factors on behavior and mental processes.