Final answer:
The rat snake represents the outgroup among the listed species because it is not a mammal and diverged earlier in the evolutionary history compared to the other species, which are all mammals and share a more recent common ancestor.
Step-by-step explanation:
The question asks which of the listed species represents the outgroup of the others. An outgroup in a phylogenetic analysis is a species or group of species that is presumed to have diverged before the species that make up the rest of the taxa being studied. In this scenario, based on the provided classifications, the rat snake would represent the outgroup as it is not a mammal like the rest of the listed species, which include the elephant, flying squirrel, kangaroo, platypus, and sugar glider. These mammals share a more recent common ancestor with each other than they do with the rat snake, making the snake the outgroup of this set.