The correct answer is:
Walton's letters speak of his thirst for knowledge and his loneliness, and they introduce the novel's main themes.
The whole novel is basically told through these letters, so they are used to advance the plot an offer some sense of authenticity to an implausible story. Plus, it is a way for them to express their own feelings and thoughts about the whole thing.
Explanation:
Captain Walton gives the frame story in Frankenstein, the fictional epistolary means by which the complete story is designed and written. Walton corresponds with his sister, describing how he found Victor Frankenstein on the Arctic ice, and what effect Frankenstein's story (and later the Monster's story) had on him. Writing it down also presents Walton to study and reflect on Frankenstein's life in detail .
