Final answer:
The most important variable for changing a person's body composition is caloric balance—calories consumed compared to calories burned.
Step-by-step explanation:
The SINGLE MOST important variable with regard to maintaining or changing a person's body composition is c. Calories in/calories out. The balance of caloric intake versus caloric expenditure is foundational for body weight and composition management. If caloric intake exceeds what is used through metabolism and physical activity, the body stores the excess energy as fat. Conversely, consuming fewer calories than the body burns leads to the usage of stored fat for energy. While exercise, genetics, the composition of foods, and meal timing play roles in body composition, the principle of energy balance is the most significant factor influencing changes in weight. If the number of calories consumed exceeds the number of calories burned, the body will store excess energy as fat, leading to weight gain. On the other hand, if the number of calories burned exceeds the number of calories consumed, the body will use stored fat for energy, leading to weight loss.