Answer:
Billy and Tanya's decision to plan their first child about a year after they get married because many of their friends have done that is most consistent with the social influence theory.
Social influence theory posits that people's attitudes and behaviors are influenced by the social context in which they find themselves. This can include the influence of friends, family, and other social groups, as well as societal norms and cultural expectations.
In the case of Billy and Tanya, their decision to plan their first child after a year of marriage may be influenced by the social norms and expectations of their peer group, as well as the influence of their friends who have already started families. This is an example of social influence theory because their behavior is being influenced by the social context in which they find themselves.