Polar bears maintain homeostasis in freezing cold weather primarily through their thick fur, which retains heat. Additionally, they have a layer of fat (blubber) beneath the skin that provides insulation and helps to keep them warm in extreme cold. Therefore, the most appropriate response given your options is:
Their thick fur retains heat.
While their metabolism does play a role in overall energy management, the specific mechanisms for maintaining body temperature in the cold environment are largely attributed to their fur and blubber.