Researchers manipulate or control variables in order to conduct?

D) A true experiment

In a true experiment, researchers manipulate or control variables to establish cause-and-effect relationships. This contrasts with other methods like case studies or naturalistic observation, where the researcher doesn’t control the variables but rather observes them in a natural setting. The double-blind procedure is a specific technique often used in experiments to reduce bias, but it is not the general method of manipulating variables.