Final answer:
The "indiscernible barbed wire between" Gatsby and Daisy refers to the social class difference between them, which creates a barrier in their relationship. Therefore, the correct option is D. Social class.
Step-by-step explanation:
The "indiscernible barbed wire between" Gatsby and "such people" as Daisy refers to the social class difference between them. Gatsby, despite his wealth and success, comes from a lower social class background, while Daisy belongs to a higher social class. This difference in social class creates an invisible barrier or wire between them, making it difficult for them to have a meaningful relationship.