Final answer:
A) a controlled experiment best matches the NIH definition of a clinical trial, as it involves the deliberate intervention to evaluate effects on health outcomes, unlike observational studies, surveys, or case studies.
Step-by-step explanation:
The best match for the National Institutes of Health (NIH) definition of a clinical trial is not an observational study, survey research, or a case study. The NIH defines a clinical trial as a controlled experiment that involves research participants and is designed to evaluate the effects of interventions on biomedical or health-related outcomes. Therefore, the correct answer is A) a controlled experiment.
An observational study measures characteristics in a sample without attempting to manipulate variables of interest, and hence does not fit the definition of a clinical trial. Similarly, survey research and case studies do not involve the active intervention that characterizes a clinical trial. These other types of studies may be useful for generating hypotheses or understanding associations, but they do not test the efficacy or safety of medical interventions the way clinical trials do.