What best defines a positive control in an experiment?

A positive control in an experiment is defined as a treatment or condition that is expected to produce an effect and is used to validate that the experimental setup is capable of detecting the effect being tested. By using a treatment with a known response, researchers can confirm that the experiment is functioning properly. This also allows them to compare the results of the experimental conditions with the expected outcomes from the positive control, ensuring the experimental conditions are adequate.

In contrast, options that describe negative outcomes, lack of response, or unmanipulated groups do not provide the necessary benchmark to assess the validity and functionality of the experiment. In summary, the importance of a positive control lies in its ability to reinforce the credibility of the experiment by establishing that the methods used can generate predictable, reliable results.