The correct option is A interdependent
The line "Etiquette is dependent on culture" means that etiquette is dependent on culture. At the same time, culture is also dependent on etiquette, because in a way the etiquette defines a culture, as mentioned here: "Etiquette reflects formulas of conduct in which society or tradition have invested."
Hence the most suitable word is 'interdependent', which means ''dependent on each other'. So, A is the correct option.
Option B: 'Dependents' is generally used for a person who relies on someone for some kind of aid. Hence B is incorrect.
The passage focuses on the relation between etiquette and culture, and how one affects another. Hence the words 'excellent' and 'widely different' are not relevant to the passage. So, options C and D are incorrect.