Final answer:
The unique aspect of the story is that it involves a Greek king and explores themes of loyalty, virtue, and the nature of belief and storytelling.
Step-by-step explanation:
The unique aspect of the story discussed from Andrew Lang's “The Story of the Greek King and the Physician Douban” is b) The story involves a Greek king. The passage you provided outlines a complex narrative where loyalty, virtue, and jealousy are central themes, as well as the power of storytelling itself. It raises the idea that one should not hastily believe accusations without proof, referencing a tale within a tale where a king is advised not to trust a mother-in-law's words without question.