Explanation
Correct answer: C.
Choice C is the best answer because it most logically completes the text’s discussion of Bastos’s study of a wild kea parrot. In this context, "accidental" means unplanned or unintentional. The text first describes Bastos’s study, which concerns a kea that is observed using small stones to preen its feathers. The text then mentions colleagues who are skeptical (that is, they have doubt) about Bastos’s findings, and finally describes how Bastos and her team responded to the skepticism of those colleagues. Given that the colleagues mentioned in the text expressed skepticism regarding Bastos’s findings, the best answer choice must be one that completes the text in a manner such that the skeptics’ opinion regarding the kea’s use of stones disagrees with that held by Bastos and her team. Since Bastos and her team showed that the kea’s use of stones was deliberate (that is, intentional), the skeptics’ opinion in this context must be that the kea’s use of stones was unintentional, or accidental.
Why the other choices are wrong
Choice A
Choice A is incorrect because the best answer choice is one that portrays skepticism, or doubt, of Bastos’s claim that the kea’s usage of stones was deliberate, or intentional. If the skeptics found the kea’s usage of stones "intriguing," or fascinating, this would not be at odds with the position of Bastos and her team; in fact, it is reasonable to believe that someone who agreed that the kea’s stone usage was deliberate would also find it intriguing.
Choice B
Choice B is incorrect because if the skeptics believed that the kea’s usage of small stones was "obvious," or evident, this would not be contrary to the observation of Bastos and her team that the kea’s usage of stones was deliberate: in fact, these opinions would be consistent with each other.
Choice D
Choice D is incorrect because if the skeptics believed that the kea’s usage of small stones was "observable," or visible, this would not conflict with the claim of Bastos and her team that the kea’s usage of stones was deliberate: instead, these positions would agree.