Explanation
Correct answer: D.
Choice D is the best answer because it most logically completes the discussion about the genealogy of Arthurian legends. The text establishes a chronology of Arthurian sources, with Vita Sancti Cadoci composed in the 11th century (the 1000s), Geoffrey of Monmouth’s History written in the 1130s, Wace’s Roman de Brut in 1155, and Culhwch and Olwen in the second half of the 12th century (1150–1200). Importantly, the text indicates that along with adapting Geoffrey’s History, Wace added "the famous Round Table" to it. If Wace introduced the idea of the Round Table in 1155, it logically follows that this element was not present in earlier works. This suggests that the Arthurian legends that informed Vita Sancti Cadoci, which was written before both Geoffrey’s and Wace’s works, would not have featured the Round Table.
Why the other choices are wrong
Choice A
Choice A is incorrect because the text discusses several works that include legends about a character, King Arthur, but doesn’t address the historical accuracy of the stories in those works or suggest that the creation of Culhwch and Olwen was relevant to any other works’ accuracy.
Choice B
Choice B is incorrect because the text doesn’t suggest that works containing stories of the Round Table were available to Geoffrey of Monmouth when he composed his History in the 1130s, whether he was aware of them or not; it instead suggests that the idea of the Round Table wasn’t introduced until more than a decade later, when Wace added it to Geoffrey’s History.
Choice C
Choice C is incorrect. Although the text indicates that the Arthurian legends in Geoffrey of Monmouth’s History differ from those in Wace’s Roman de Brut in that Wace added the idea of the Round Table, the text doesn’t compare the overall content of the Arthurian legends in the two works and doesn’t indicate anything about the overall content of the legends in Culhwch and Olwen.