Which of the following best describes a 'placebo'?

A 'placebo' is typically understood as a substance or treatment that has no therapeutic effect but can lead to psychological benefits for some patients. This is primarily because the act of taking something—often a sugar pill or saline injection—can trigger responses in the brain that may alleviate symptoms, highlighting the power of belief and expectation in the healing process.

The definition aligns best with the notion of something that produces a positive mental effect without having a direct physical impact, which is why option B is the correct choice. This ties into the concept of the placebo effect, where individuals may feel better simply because they believe they are receiving treatment.

While the other choices might touch on aspects of placebos, they do not capture the complete essence. For instance, a 'medication with no known effects' does not necessarily convey the psychological aspect vital to placebos, and stating that a treatment 'worsens conditions' is antithetical to the purpose of a placebo, which aims to promote well-being. Moreover, a 'drug with guaranteed efficacy' contradicts the very definition of a placebo, which is not aimed at having any measurable pharmacological effect.