Final answer:
Beavers, living in freshwater environments with excess water, have adaptations like efficient water reabsorption (option c) allowing them to excrete dilute urine, while humans have adaptations for more regulated urine concentration due to varying water availability.
Step-by-step explanation:
Beavers have relatively little need to conserve water and could therefore be expected to have efficient water reabsorption than humans do. Beavers live in freshwater environments where they are in danger of osmotic water gain. Their kidneys are adapted to produce dilute urine, allowing them to excrete excess water without losing too many electrolytes. Unlike desert-dwelling animals which have evolved to have highly concentrated urine to conserve water, beavers have adapted to their abundant water surroundings with different renal physiology.
In contrast to beavers, humans have to regulate their water balance more carefully, as they do not live in an environment with an excess of water. Human kidneys, therefore, do not tend to produce as dilute urine as a beaver's kidneys would. The juxtamedullary nephrons in human kidneys have a longer loop of Henle, which is an adaptation allowing the regulation of urine concentration suitable for an environment where water balance is more crucial.