Choice D is the best answer. The previous question asks what can most reasonably be inferred about guppies based on the experiments in Trinidad described in the passage. The answer, that some guppy genetic traits will evolve more readily than others, is best supported by the last sentence of the passage: "Other studies of guppies in Trinidad have shown evolutionary change in as few as two and a half years, or a little over four generations, with more time required for genetic shifts in traits such as the ability to form schools and less time for changes in the colorful spots and stripes on a male's body."
Choices A, B, and C are incorrect because the cited lines don't support the answer to the previous question. Instead, they pose a question central to Reznick's research (choice A), describe the conditions that led Reznick to consider conducting experimental evolution tests in Trinidad (choice B), and describe how Reznick's team removed guppies from one area and reintroduced them in another (choice C).