The factor of political socialization that is considered to be the best predictor of someone's political leanings is (D) family.
Family is the primary source of socialization during childhood, and children often adopt the political beliefs and values of their parents or other family members. Family values, traditions, and experiences can shape an individual's political ideology, and studies have shown that individuals who grow up in households with strong political identities are more likely to adopt those identities themselves.
While other factors such as religious affiliation, peers, and schools can also have an impact on an individual's political leanings, family is considered to be the most influential and consistent predictor of political socialization.