Which component contributes most significantly to the basal metabolic rate?

The basal metabolic rate (BMR) is primarily influenced by body composition, specifically the amount of lean muscle mass a person possesses. Muscle tissue is metabolically active and requires more energy to maintain than fat tissue. As a result, individuals with a higher muscle mass will generally have a higher BMR.

Factors like physical activity levels and the thermic effect of food do play roles in overall energy expenditure, but they do not contribute as significantly to BMR itself. Physical activity affects total daily energy expenditure more than it does BMR, while the thermic effect of food involves the energy used during the digestion and absorption of food, which occurs after eating rather than at rest.

Temperature regulation can impact metabolic processes, but its influence on BMR is comparatively minor. Overall, the significant contribution of body composition, particularly muscle mass, to the basal metabolic rate is why this option is the most accurate in the context of understanding energy expenditure at rest.