Final answer:
The best behavior predictor for starting an exercise program is past exercise habits. They often have a greater impact on current actions compared to genetic factors, body weight, or dietary preferences due to established routines and behavior patterns.
Step-by-step explanation:
The best predictor of a person's likelihood to begin participating in an exercise program with respect to behavior is a) Past exercise habits. Past behaviors, such as earlier involvement in physical activity, typically have the strongest influence on current actions due to established routines and psychological factors like habituation and comfort with the activity. Genetic factors, body weight, and dietary preferences can influence a person's overall health and propensity to exercise, but they do not predict actual behavior as reliably as previous exercise habits. Understanding an individual's history with physical activity can provide insights into their current motivation and likelihood to engage in exercise.