Explanation
Correct answer: D.
Choice D is the best answer because it most logically completes the text’s discussion of the costs of studies. The text indicates that HILDA is a longitudinal study—it involves repeated observations over a long time—and that the "need for years of data collection" is what causes it and most other longitudinal studies to have high costs. The text then contrasts such studies with more straightforward studies, using the example of a fitness study that wants only to find out how many regular exercisers in a particular location do weight training, that may not demand a large budget. The contrast emphasized in the text most directly suggests that the fitness study may cost less than a study like HILDA because the fitness study aims only to determine a current percentage and thus can be completed well without extending data collection over years.
Why the other choices are wrong
Choice A
Choice A is incorrect because the text explicitly introduces the idea of a less expensive, straightforward fitness study as a contrast to many longitudinal studies that, like HILDA, have high costs because they require data collection over many years. The fitness study is described as trying merely to gather information about a current behavior (how many regular exercisers do weight training) rather than track trends over time, which suggests that costs could remain low because expensive longitudinal methods would not be needed, not because they would be suitable.
Choice B
Choice B is incorrect because the idea that HILDA researchers could easily add questions to their study might be relevant to the costs of that survey but would be completely irrelevant as an explanation for why an unrelated fitness study may not need a large budget; the studies have no direct bearing on each other.
Choice C
Choice C is incorrect because the idea that HILDA involves more people than necessary would be relevant to the costs of that survey but would be completely irrelevant as an explanation for why an unrelated fitness study may not need a large budget; the studies have no direct bearing on each other.