Final answer:
The most logical reason for the size difference in a species of fish in two ponds is that the fish in the large pond have more predators, so they have to grow larger.
Step-by-step explanation:
The most logical reason for the size difference in a species of fish in two ponds is D. The fish in the large pond have more predators, so they have to grow larger. When fish are exposed to a higher number of predators, those that grow larger have a better chance of survival because they are less likely to be preyed upon. Over time, this leads to a population of larger fish in the pond with more predators.