Final answer:
Among the options provided, the desert cactus with thin, sharp needles as an adaptation to prevent water loss is an example of a trait that increases fitness by enhancing survival in a harsh, dry environment.
Step-by-step explanation:
Adaptations are heritable traits that enhance an organism's ability to survive and reproduce within their particular environment. An example of such an adaptation is a desert cactus with thin, sharp needles instead of leaves, which helps to prevent water loss in an arid climate. This trait increases the fitness of the cactus by allowing it to conserve water and survive long periods of drought. In contrast, physical features such as white fur would not be adaptive in a tropical rainforest where camouflage within the green environment is more advantageous for survival. Similarly, the specialized diet of a bird eating only corn would not be conducive to its long migratory journey across the Pacific Ocean where other food sources are necessary. Meanwhile, a hunting dog's ability to perform circus tricks is not an adaptation that increases its natural survival or reproductive success; such a skill is artificially selected by humans and is extraneous to the dog’s fitness in its natural environment.
Certain traits like mimicry and camouflage provide a survival advantage. For instance, animals like the tropical walking stick can blend in with their environment to evade predators, which is a good example of an adaptive trait. Moreover, Darwin's finches demonstrate how a change in beak shape, driven by available food sources, is another example of natural selection leading to an adaptation that directly impacts survival and reproductive success.