Final answer:
The least likely reason for Professor Sandburg to track adolescent population size over time would be to analyze the influence of peer groups on adolescent behavior, as this is more related to social dynamics rather than demography.
Step-by-step explanation:
The question explores why a social scientist, particularly Professor Sandburg who works with advocacy groups, would be interested in tracking the size of the adolescent population over time and identifies which of the provided options is least likely to be a reason for such interest. Among the options presented, analyzing the influence of peer groups on adolescent behavior is the least relevant to tracking population size over time. This option is more concerned with the social dynamics within the adolescent group rather than the demographic information about the population itself. The other options such as understanding demographic shifts in advocacy group membership, assessing the impact of social policies on the adolescent population, and identifying trends in educational attainment among adolescents are more directly influenced by demographic data, making them more relevant for a social scientist studying population trends.