Final answer:
The placebo effect refers to changes in a participant's behavior or condition due to their expectations regarding the treatment, even if the treatment is inert, such as a sugar pill. It demonstrates the psychological impact of receiving a treatment in a research study.
Step-by-step explanation:
The placebo effect means C. the expectations of the participants influence their behavior. This phenomenon occurs when the beliefs or expectations of participants in an experiment influence their experience or outcomes, even if the treatment they receive is inert. In research studies, a placebo is often used in a control group to help researchers isolate the effects of the treatment from the psychological impact of receiving a treatment. Proper experimental design, such as double-blind studies, is crucial for mitigating both experimenter and participant expectations and biases, ensuring that the results are attributable to the treatment itself rather than psychological effects.