Final answer:
Option C is correct; Le Morte d'Arthur by Sir Thomas Malory is a compilation of the King Arthur legends that existed in oral tradition before being written down in the 15th century.
Step-by-step explanation:
The statement that Le Morte d'Arthur is a written version of legends that had long been shared through the oral tradition is a true statement. Specifically, option C is correct. Le Morte d'Arthur was written by Sir Thomas Malory in the 15th century and compiles many of the King Arthur legends which had been circulating for centuries through oral storytelling into a cohesive narrative. This work is not the original source of all King Arthur stories as they predate the text, nor is it a contemporary adaptation as it was written in medieval times.