Final answer:
An experimental psychologist would not give a diet consisting only of fast-food to Group C: Athletes aged 20-25 because a fast-food diet is unlikely to align with their nutritional needs and may have negative effects on their performance and overall well-being.
Step-by-step explanation:
To determine how a fast-food diet affects the moods of individuals, an experimental psychologist would not give a diet consisting only of fast-food to Group C: Athletes aged 20-25. Athletes generally follow a specific diet to optimize their performance, which includes a focus on nutritious and balanced meals. A fast-food diet is unlikely to align with their nutritional needs and may have negative effects on their performance and overall well-being.