Final answer:
A controlled experiment is the purest form of an experiment because it allows accurate manipulation of the independent variables and control over the experimental setting.
Step-by-step explanation:
The purest form of an experiment is A. a controlled experiment. This is because it allows the researcher to manipulate the independent variables, randomize participants into conditions, and control features of the experimental setting with precision. In a controlled laboratory environment, researchers can execute a study purposefully designed to investigate cause-and-effect relationships by modifying the independent variable(s) and observing the effect on the dependent variable(s) while maintaining strict control over extraneous factors. This contrasts with natural or field experiments where variables are observed in their natural setting without intervention, and while these provide ecological validity, they often lack the same level of control as laboratory experiments.