The correct answer is option 2: fat. Polar bears have a thick layer of fat, called blubber, underneath their skin that helps them stay warm in their cold Arctic environment. Blubber acts as an insulator, providing a barrier between the bear`s body and the cold water or air. It helps to retain body heat and prevents it from escaping into the environment. The more blubber a polar bear has, the better it is able to maintain its core body temperature. This layer of fat also provides buoyancy, allowing the polar bear to float easily in water. While options 1 (muscle), 3 (cartilage), and 4 (hair) are all present in polar bears, they do not serve the primary function of keeping the bear warm.