Explanation
Correct answer: D.
Choice D is the best answer because it most logically completes the text’s discussion of the relationship between habitat latitude and body shape in saltwater fish species. In this context, “concentrated among” means largely confined to. The text sets up a contrast between tropical species, which are generally characterized by a single shape—deep-bodied forms—and polar and temperate species, which are “highly dispersed across the morphological spectrum” (meaning they exhibit a wide range of body shapes). This context supports the idea that tropical species are concentrated among deep-bodied physical forms—that is, largely confined to this single morphological category rather than spread across many.
Why the other choices are wrong
Choice A
Choice A is incorrect because saying that tropical species are “authenticated by,” or proven genuine by, deep-bodied physical forms wouldn’t make sense. Instead, the text describes the typical morphology (body shape) of tropical fish species and how that differs from the typical morphologies of polar and temperate fish species.
Choice B
Choice B is incorrect because saying that tropical species are “habituated to,” or accustomed through repeated exposure to, deep-bodied physical forms wouldn’t make sense. Instead, the text describes the typical morphology (body shape) of tropical fish species and how that differs from the typical morphologies of polar and temperate fish species.
Choice C
Choice C is incorrect because saying that tropical species are “contemporary with,” or existing at the same time as, deep-bodied physical forms indicates that the species exist at the same time as their body shapes do, which, if it is a meaningful statement at all, is at best trivially true and unrelated to the text’s discussion of the typical morphology (body shape) of tropical fish species and how that differs from the typical morphologies of polar and temperate fish species.