_________ rule states that mammals from colder climates generally have shorter ears and limbs to minimise heat loss.

The mammals from colder climatic regions have extremities (ear, snout, tail, legs) shorter than those of warmer regions, this is called as Allen's rule. For other options, Rensch's rule states that birds have narrow wings in cold areas as compared to those of warmer regions.
Bergman's rule states that warm blooded animals (birds, mammals) of colder region are of larger size as compared to those of warmer areas.
Jordan's rule states that fishes in areas of low temperature tend to have more vertebrae as compared to those in warm water.