Choice A is the best answer. The first two paragraphs of the passage describe the physical process by which the Venus flytrap closes its trap but also note certain long-standing questions about that process: "How does the plant encode and store the information from the unassuming bug's encounter with the first hair? How does it remember the first touch in order to react upon the second?" The passage then answers those questions by discussing, in the third and fourth paragraphs, a study conducted by Dieter Hodick and Andreas Sievers that identified the physiological means behind the closing of the Venus flytrap's trap and, in the last paragraph, a study conducted by Alexander Volkov that confirmed and built on Hodick and Sievers's findings. The primary purpose of the passage can therefore be seen as discussing scientific findings that explain how the Venus flytrap closes its trap.
Choice B is incorrect because the passage doesn't discuss the Venus flytrap's ability to close its trap in the context of the abilities of other plants. Choice C is incorrect because the passage discusses how the closing action operates but not how it has evolved. Choice D is incorrect because the passage doesn't provide an overview of the Venus flytrap and its predatory behavior; it merely notes in passing that the closing action has a predatory function.