Final answer:
Arthur does not know of his destiny to be king because he is unaware that he is the adopted son of Ector and the biological son of Uther Pendragon. His identity was concealed by Merlin for his safety and it was not until later that he discovered his royal heritage and his destiny.
Step-by-step explanation:
The question pertains to the legendary character King Arthur, and specifically why Arthur does not know that he is destined to become king. The correct answer to the question is D) he doesn't know that he is Ector's adopted son and that Uther is his biological father. According to Arthurian legend, Uther Pendragon, Arthur's biological father, entrusted Merlin to hide his son's identity to keep him safe. Merlin placed Arthur with Sir Ector, who raised Arthur as his own, without revealing his royal lineage. Only later did Arthur learn of his true parentage and destiny to be king.