Which adaptation would be most to animals living in the tundra

Thick fur, padded feet, thick skin et cetera.

Explanation:

The animals that live in the tundra region needs to keep their body warm. The climate their is freezing and the body of animals living in those region have some adaptations that keeps them warm.

The vegetation their is limited so they feed mostly on the meat and animal fat to keep to their body.

The animals have hairy and furry skin which acts as a insulator and help to keep the body warm.

i believe it is "they are not used up by reactions" as the answer to your question, text me back if this is correct because i just learned this.