Final answer:
The evolutionary phenomenon where predator and prey species continually adapt in response to each other is known as coevolution. The best example of the Red Queen Hypothesis, where species must constantly evolve to maintain fitness, is the extinction of a snail population after a parasitic invasion due to its asexual reproduction limiting genetic variation.
Step-by-step explanation:
When predator and prey species acquire new adaptations to help them survive in the constant "arms race" between them, the evolutionary phenomenon occurring is coevolution. This arms race is a dynamic process where prey may evolve defenses or survival strategies, and predators develop new methods to bypass these defenses, illustrating reciprocal evolutionary changes. The scenario that best supports the Red Queen Hypothesis—where organisms must continually evolve to maintain their relative fitness amidst constant evolutionary change—is b. Individuals of a snail population that reproduce asexually die out after a parasite invades its territory. This is because the lack of genetic variation due to asexual reproduction hampers the snail population's ability to evolve rapidly in response to new threats compared to sexually reproducing organisms, thus they cannot "run" to stay in the same place in an evolutionary context.