Which of the following factors primarily affects basal metabolic rate (BMR)?

The basal metabolic rate (BMR) is influenced by multiple factors, and in this case, gender, age, and physical activity level all play significant roles in determining an individual's BMR.

Gender impacts BMR primarily due to differences in body composition between males and females. Generally, males tend to have a higher BMR because they typically have a greater proportion of muscle mass compared to females, who tend to have a higher fat percentage. Muscle tissue requires more energy to maintain than fat tissue, leading to differences in metabolic rates.

Age also significantly contributes to BMR. As individuals age, there is a natural decline in muscle mass and potentially an increase in body fat. This shift can lead to a decrease in BMR because muscle is metabolically more active than fat tissue. Therefore, older adults often experience a lower BMR compared to younger individuals.

Physical activity level affects BMR as well, but more indirectly. Regular exercise can increase muscle mass, which can raise BMR even at rest. However, while this is an important factor, it does not directly dictate BMR like gender and age do. Instead, it's more about how the activity can influence body composition and therefore impact BMR over time.

Considering all three factors together, it is clear that