Questions: The primary factor determining basal metabolic rate is: activity levels, age, sex, fat-free mass.

The answer is the fourth one (D): fat-free mass.

Explanation for each option:

1. Activity levels: While physical activity can influence overall energy expenditure, it is not the primary factor determining basal metabolic rate (BMR). BMR is the amount of energy expended while at rest in a neutrally temperate environment, and it does not account for physical activity.

2. Age: Age does affect BMR, as metabolic rate generally decreases with age. However, it is not the primary factor. The decline in BMR with age is often due to a decrease in fat-free mass and changes in hormone levels.

3. Sex: Sex can influence BMR, with males typically having a higher BMR than females, primarily due to differences in body composition, such as higher muscle mass in males. However, sex itself is not the primary determinant.

4. Fat-free mass: Fat-free mass (which includes muscle, bone, organs, and water) is the primary factor determining BMR. Muscle tissue, in particular, is metabolically active and requires more energy to maintain than fat tissue. Therefore, individuals with a higher fat-free mass generally have a higher BMR.

Summary: The primary factor determining basal metabolic rate is fat-free mass. This is because muscle and other components of fat-free mass are metabolically active and require more energy to maintain, thus significantly influencing BMR.