Final answer:
The control group would not be given a fast-food-only diet in an experiment designed to study the effects of fast food on mood, serving as a baseline for comparison.
Step-by-step explanation:
In an experimental setup where an experimental psychologist is interested in determining how a fast-food diet affects the mood of individuals, the group that would not be given a diet consisting only of fast food is the control group. The purpose of the control group is to serve as a baseline by which the effects on the experimental group, which receives the fast-food diet, can be compared. The design typically ensures that the control group is identical to the test group in every way possible, except for the exposure to the independent variable, which, in this case, is the fast-food diet.