Explanation
Correct answer: D.
Choice D is the best answer. “Thus” logically signals that the claim in this sentence—that animals performing only basic actions should allocate relatively few resources to their brain tissue—is a consequence of the previous sentence’s claim about the energy demands of animal brains (namely, that the more diverse an animal’s behaviors, the more energy its brain needs).
Why the other choices are wrong
Choice A
Choice A is incorrect because “subsequently” illogically signals that the claim in this sentence occurs later in a chronological sequence of events than the previous sentence’s claim about the energy demands of animal brains. Instead, the second claim is a consequence of the first.
Choice B
Choice B is incorrect because “besides” illogically signals that the claim in this sentence provides a separate point in addition to, or apart from, the previous sentence’s claim about the energy demands of animal brains. Instead, the second claim is a consequence of the first.
Choice C
Choice C is incorrect because “nevertheless” illogically signals that the claim in this sentence is true in spite of the previous sentence’s claim about the energy demands of animal brains. Instead, the second claim is a consequence of the first.