Answer: The option that best demonstrates natural selection is:
Shelley mates two breeds of sheep to try to produce a hardier breed.
Explanation: This is an example of artificial selection, which is a process similar to natural selection but is intentionally done by humans to achieve certain desirable traits in a population. In this case, Shelley is selecting certain traits by intentionally mating two breeds of sheep to produce offspring with a combination of traits that make them better adapted to their environment. Over time, the hardier breed will become dominant in the population due to natural selection.