Final answer:
Option (D), Curzon calls Isabel "Miss", which is a polite form of address for a young lady during the time period of the novel.
Step-by-step explanation:
In the book, Curzon refers to Isabel as "Miss". While Curzon and Isabel share a close relationship, and Curzon does show respect and camaraderie towards her, the term "Miss" is used by him as a form of address that is both polite and indicative of Isabel’s status as a young lady. This terminology is reflective of the time period in which the novel is set, as it denotes a form of respect for a female peer.