Final answer:
Daisy and Nick's exact relationship is unknown based on the given information.
Step-by-step explanation:
In the novel, it is not explicitly stated how Daisy and Nick are related. However, based on the given information, it can be inferred that they are not related by blood or marriage. Daisy's mother mentions that they are not friends, and when asked about their relationship, Daisy refers to Mr. Giovanelli as an intimate friend. Therefore, option D, they are cousins, can be eliminated.
Without further information, it is not possible to determine if they are brother and sister (option A) or step-siblings (option C). It is also important to note that Daisy's mother states she never spoke to them, suggesting that they are not closely related.
To summarize, based on the given information, it can be concluded that Daisy and Nick are not brother and sister, married, step-siblings, or cousins. Their exact relationship remains unknown.