The biggest influence on one’s marital success can vary from person to person, but research often points toward factors such as communication, compatibility, and shared values rather than the specific options presented.
However, if I had to choose from the provided responses, many studies suggest that a friends’ relationship with spouse (option a) may have a significant influence because social circles can impact behavior, attitudes, and relationship satisfaction. The support and example set by friends' relationships can also contribute to one's own marital dynamics.
That said, consider that effective communication, emotional support, and shared goals are often more critical for marital success than the elements listed here.