Explanation

The results reported in the last sentence of the passage suggest that hummingbirds do pollinate red-flowered plants and that hawkmoths do pollinate white-flowered plants. But to prove that they are attracted preferentially to those colors requires knowing whether they also pollinate flowers of the other color that are uncovered during their respective foraging hours—white flowers uncovered during hummingbirds’ daytime hours, and red flowers uncovered during hawkmoths’ nocturnal hours. Choice D states that such plants remain unpollinated, so it is the correct answer. Of the other options, both Choices C and E could suggest the opposite of what is required, so they are incorrect. Choice A is incorrect because the comparative frequency at which uncovered gilia flowers are pollinated has no bearing on the argument. Likewise, the presence of some quantity of unpollinated flowers of either color does not affect the argument, so Choice B is also incorrect.