Final answer:
The Green Knight comes into Arthur's castle during Christmas.
Step-by-step explanation:
The Green Knight comes into Arthur's castle during Christmas time, so the correct answer is a) Christmas. In the medieval poem Sir Gawain and the Green Knight, the story takes place in the Arthurian court and is set during the Christmas season. The arrival of the Green Knight and the subsequent challenge he presents to the knights of the Round Table is a central event in the story.