All of the options listed can influence political socialization in different ways. However, if we consider the typical roles, a U.S. senator (d) might be seen as more of a political figure rather than a direct agent of socialization compared to the others, who are more likely to have direct and personal interactions that shape individual political beliefs and values. While they can influence political views, their role is more about governance and legislation rather than direct socialization. Therefore, the answer is:
d. a U.S. senator