Answer: The right answer is frame story.
Explanation: Just to elaborate a little on the answer, it can be added that a frame story, or frame narrative, is a literary device and it occurs when the character in a literary work becomes the narrator of another story, enriching the narrative and further engaging the reader. The Canterbury Tales by Chaucer is a good example of this technique. In other instances, there are various narrators in a story, thus providing the readers with alternative perspectives - this occurs in Conrad's Heart of Darkness and in Samuel Taylor Coleridge's "The Rime of the Ancient Mariner," for example.