Final answer:
The placebo effect in the described study is produced by the participants' belief in the treatment, not the drug, the shape of the pill, or the inert ingredients. It demonstrates how expectations can influence health outcomes, especially in a double-blind study designed to reduce bias.
Step-by-step explanation:
In a research study where one group is given Drug X and another group is given a placebo to treat depression symptoms and both groups report a reduction in symptoms, what produced the placebo effect is c) The participants' belief in the treatment. The placebo effect occurs when a participant's expectations influence their experience and symptoms. It is a psychological response where the mere belief that one is receiving treatment can result in feeling better, despite no active pharmaceutical ingredient being present in the placebo.
Researchers administer a placebo to control for the expectations that might influence the experimental outcomes. This is why a double-blind study is often used, so neither the participants nor the researchers know who is receiving the actual medication and who is receiving the placebo, reducing bias from both participants and experimenters.