What effect does Shelley create at the end of the novel by returning to Walton’s letters to his sister?

Shelley creates mystery by using Walton's voice at the end of the book.

Shelley creates a new beginning by returning to the start of the book.

Shelley creates closure by ending the novel in the same way that it began.

Shelley creates suspense by leaving the voice of Victor out of the end of the novel.