Final answer:
The best predictor of an enduring and happy marriage between the ages of 75 and 80 according to Vaillant's research, is emotional intelligence.
Step-by-step explanation:
According to the research of Vaillant the characteristic in middle age that is the best predictor of having an enduring and happy marriage between the ages of 75 and 80 is C. emotional intelligence. Emotional intelligence encompasses the ability to recognize, understand, and manage our own emotions, as well as to recognize, understand, and influence the emotions of others. This attribute is essential in navigating the complexities of marital relationships and contributes to mutual understanding, empathy, and conflict resolution, which are paramount for a long-lasting, happy marriage.