Answer: The correct answer is D. Homeostasis.
Explanation: A polar bear's ability to maintain a stable internal body temperature in cold conditions is an example of homeostasis, which is the characteristic of all living things that enables them to maintain a stable internal environment despite changes in the external environment. This involves the regulation of various physiological processes, such as body temperature, blood pH, and glucose levels, to name a few. In the case of polar bears, they are able to maintain a stable internal body temperature in cold conditions by several mechanisms, such as having thick fur to insulate themselves, a layer of fat under their skin to retain heat, and the ability to regulate their metabolic rate.