Final answer:
In the context of human metabolic rate, thermogenesis tends to have the smallest contribution to energy output, while basal metabolism or BMR is the largest contributor.
Step-by-step explanation:
The question pertains to the components that contribute to the total energy expenditure in the human body. The options provided are thermic effect of food, basal metabolism, physical activity, and thermogenesis. Of these components, the smallest contributor to energy output is generally thermogenesis, which is the production of heat in the body, not including the basic functions or physical activity. Basal metabolism or the basal metabolic rate (BMR) accounts for the largest share, approximately 70% to 75%, of daily energy expenditure and is used for the body's basic functioning at rest. A person's BMR is influenced by factors such as age, gender, total body weight, and muscle mass. Physical activity accounts for about 20% of energy expenditure, and the thermic effect of food, which refers to the energy required for digestion, absorption, and disposal of ingested nutrients, is around 10%.