Answer Choice A is the correct answer, because the text primarily concerns the discovery of lignin in the red algae Caliiarthron cheilosporioides, discusses some possible implications of this discovery including, “that a biological building block fundamental to the success of land plants has roots that stretch back far deeper — and maybe wider — through evolutionary time than was known.” Answer Choice B is incorrect because the passage doesn’t consider generally the differences among specialists in different fields of science, like a molecular biologist and an astrophysicist. Answer Choice C is incorrect because it’s not right to call the phenomenon and presence of lignin in algae as “particularly vexing” and because the Passage supports multiple explanations. Answer Choice D is incorrect because the Text doesn’t indicate ways that the presence of lignin in organisms “can be exploited.”
How to solve this? The Question asks, “The passage is primarily concerned about [...]?” Because this is a [Big Picture] Question, we might be better off solving it after solving the other related questions, so that we can have a more general picture of the Passage at large and use it to guide us. If we approach it directly, however, we can look for a few key points that indicate the primary focus of the Passage. The heading, for instance, tells us that the passage is titled “A Tangled Tale of Plant Evolution,” which gives us the cue that this passage is about the evolution of plants. The first Paragraph describes lignin and its properties, the second Paragraph tells us that a new discovery has found it in algae, and the third Paragraph tells us that this finding suggests that “a biological building block fundamental to the success of land plants has roots that stretch back far deeper” into evolutionary time. The rest of the Passage continues to discuss how lignin was found, and some hypotheses about its evolutionary origins. It’s fair to say in advance, then, that the Passage concerns lignin, its discovery and some of the consequences of that discovery.
Answer Choice A says that the Passage is concerned with, “narrating how a finding was arrived at indicating possible implications.” This does match our text, although “indicating possible implications” has a moderate burden of proof which we should remind ourselves to check. But, for now, we can keep this option. Answer Choice B says that the Passage is concerned with, “explaining some differences among specialists in different fields of science.” This answer choice is incorrect because the Text doesn’t have this structure or focus, which would involve discussing differences between a biochemist and a primatologist, for example. Answer Choice C says, “identifying a particularly vexing phenomenon and endorsing a single explanation.” We should immediately note that “particularly vexing phenomenon” and “endorsing a single explanation” are “loud” claims that carry a large burden of proof. We can go back to the text to search for this representation, but should conclude that there isn’t enough evidence to support this claim (not to mention that which phenomenon that would be addressed here is unclear). Moreover, the Passage doesn’t endorse a single explanation (Lines 63-81 present two different views of the evolutionary origins of lignin in algae), so we should eliminate this option. Answer Choice D says, “describing the properties of an organism and showing how they can be exploited.” The first part of this claim seems okay, but the second part is incorrect. Going back to the Passage, we find no evidence for the ways in which lignin “can be exploited,” for either commercial or scientific reasons. Thus, we should Answer Choice A as our answer.