Answer:
A. She uses letters to introduce the main narrative.
Step-by-step explanation:
The frame story structure in Frankenstein consists of a series of narratives nested within one another. The outer frame is composed of letters written by Captain Robert Walton to his sister, which serve as an introduction to the main narrative. These letters establish the context for the story and provide a framework for the events that follow.
Within this frame, Shelley uses multiple narrators to tell different parts of the story, including Victor Frankenstein, the Creature, and others. This allows her to create tension between the narrators and to explore multiple perspectives on the events of the novel.
However, the specific structural technique that Shelley uses to introduce the main narrative is the use of letters written by Captain Robert Walton. These letters establish the setting and introduce the character of Victor Frankenstein, who is the subject of the main narrative.