Credibility is established when the descriptions, speech, actions, and events surrounding a character a. are dynamic and detailed. b. make sense in relation to the character. c. contradict in ways that interest the reader. d. cannot be easily established.

Credibility is the term that refers to something believable, some information that can be trusted. Having this in mind, credibility is established when the descriptions, speech, actions, and events surrounding a character B. make sense in relation to the character. 
It is something logical, that anyone could accept as a given.

The setting of an island might represent isolation, being far, not easy access or some kind of limitation. It also might mean frustration. As regards, her voyage to England, it might be said that it represents her growth, Annie became and adult. It might portrayed her transition from being a child to being an adult.