Final answer:
Sir Gawain learned to recognize and accept his own human frailty and limitations, as symbolized by his acceptance of the girdle from the Green Knight. Therefore, the correct option is D.
Step-by-step explanation:
The lesson Sir Gawain learned in his adventure, as told in the Arthurian stories, is best exemplified by the choice: D. He accepts the girdle from the Green Knight, not to use, but to remind him of his own frailty. This option illustrates the moral of the story, which emphasizes the recognition and acceptance of one's own human imperfections and limitations. Sir Gawain's acceptance of the girdle symbolizes his acknowledgment of his vulnerability and his humanity, a common theme in chivalric tales which often conveyed lessons of humility, honor, and the quest for personal betterment.