Final answer:
The best predictor of a long and happy life among the provided choices is close relationships, as these are consistently linked to better well-being more than IQ, genes, or socioeconomic status.
Step-by-step explanation:
Among the choices provided, the best predictor of a long and happy life is d. Close relationships. Numerous psychological studies suggest that while factors like intelligence quotient (IQ), genes, and socioeconomic status (SES) play roles in life outcomes, it is the quality of our close personal relationships that most consistently predict our happiness and longevity. For example, according to a meta-analysis by Pinquart & Sörensen (2000), factors such as socioeconomic status and network competence can influence subjective well-being in the later life. However, a Harvard Study of Adult Development found that strong relationships were a more reliable predictor of a happier and healthier life than any other factor, indicating that the social connections we nurture throughout our lives have a profound effect on our well-being.