Answer:
I believe it means that once you rech your fullest potential, you can make the imposible, posible.
It could also mean that who you are now, you used to think was impossible.
Explanation:
Theses are the two posible meaning I came up with by looking at it two different ways.
Secrets are one of the central themes in the book, although they are not the most prominent theme, they are quite important, as the ending of the book relies heavily on an untrue fact and untrue story, or rather a secret, which determines the fate of one of the characters. This quote (above in your question) particularly mirrors the quote " You never really know a person until you consider things from his point of view...until you climb into his skin and walk around it." Boo Radley remains a secret from the beginning of the book until the end and this quote relates to him as Scout and Jem try to figure him out, since he is so mysterious, yet another one of the secrets present in the novel.
“Constance Culmington, he reflected to himself, was exactly the sort of woman who would buy an island and surround herself with mystery! Nodding his head in gentle approval of his logic, Mr. Justice Wargrave allowed his head to nod... He slept..." Chapter 1, pg. 3
Answer: (C.) She is the antagonist who creates conflict to move the story forward.
