Why is Chaucer's Tales considered to be a frame narrative?

A. Each pilgrim tells a tale, which is then recorded by a scribe, who is also a character in the story.
B. Each tale told by the pilgrims is fictional, but the actual dialogue of the pilgrims themselves is assumed to be factual.
C. Each pilgrim is a character presented by the narrator, who is also a character.
D. The narrator is an actual person, but the characters he describes in each pilgrim's tale are fictional and therefore part of a frame.

Question

Why is Chaucer's Tales considered to be a frame narrative?

 		A.	Each pilgrim tells a tale, which is then recorded by a scribe, who is also a character in the story.
 		B.	Each tale told by the pilgrims is fictional, but the actual dialogue of the pilgrims themselves is assumed to be factual.
 		C.	Each pilgrim is a character presented by the narrator, who is also a character.
 		D.	The narrator is an actual person, but the characters he describes in each pilgrim's tale are fictional and therefore part of a frame.