Which of the following is a hormone responsible for regulating energy balance?

Leptin is indeed the hormone responsible for regulating energy balance. It is produced by adipose (fat) tissue and plays a crucial role in signaling the brain about the body's energy stores. When fat stores increase, leptin levels rise, which helps to suppress appetite and increase energy expenditure. This process is essential for maintaining body weight and energy homeostasis. When leptin levels are low, the opposite effect occurs, leading to increased appetite and reduced energy expenditure, which can contribute to weight gain.

The other hormones mentioned also play various roles in the body, but they do not primarily regulate energy balance in the same specific way that leptin does. Testosterone is more associated with muscle mass and libido, cortisol is a stress hormone that helps with metabolism and stress responses, and estrogen has various effects on reproductive functions and fat distribution but is not directly involved in the regulation of energy balance like leptin. Thus, the unique role of leptin in managing energy homeostasis makes it the correct choice in this context.