Which of the following is the cause of nonshivering thermogenesis in adults?

Nonshivering thermogenesis refers to increase in metabolic rate that is not a result of muscle activity. It appears to be elicited through sympathetic stimulation and circulating catecholamines. Epinephrine and norepinephrine which are released increases metabolic activity and heat generation. Ref: Guyton and Hall - Textbook of Medical Physiology, 10th Edition, Pages 821, 828-829; Medical Physiology: Principles for Clinical Medicine By Rodney A. Rhoades, 4th Edition, Page 568; Fundamentals of Human Physiology By Lauralee Sherwood, Page 489