The purpose of a control group in an experiment is option a: to serve as a standard of comparison. In an experiment, a control group is a group of subjects or samples that is not exposed to the experimental treatment or intervention. The control group is used to establish a baseline against which the experimental group is compared. By keeping all variables constant except for the manipulated variable, any differences observed between the control group and the experimental group can be attributed to the effect of the manipulated variable. This helps researchers determine the specific impact or causal relationship between the manipulated variable and the observed results. In essence, the control group provides a reference point or standard for comparison, allowing researchers to assess the effectiveness or significance of the manipulated variable.
