Question

which adaptation helps polar bears maintain a constant internal temperature (thermal homeostasis) in cold weather? three layers of fur blubber finding shade underground white skin

Explanation:

Polar bears have blubber which is a thick layer of fat. This fat layer acts as an insulator, reducing heat loss in cold - arctic environments, thus helping them maintain thermal homeostasis. Three layers of fur mainly help with water - proofing and camouflage rather than being the main heat - retaining adaptation. Finding shade underground is not a typical behavior for polar bears in their habitat. White skin is for camouflage and not a key factor in heat retention.

Answer:

blubber