To best determine where to post advertisements for a job fair, the most relevant statistic among the options provided would be unemployment rates.
Unemployment rates indicate the percentage of the labor force that is without work but available for and seeking employment. Areas with higher unemployment rates typically have a larger pool of job seekers, making them ideal locations for advertising a job fair.
While population, education, and income data can also provide useful insights, they do not directly reflect the immediate need for employment that unemployment rates do. Therefore, focusing on unemployment rates would be the most effective approach for targeting advertisement locations for a job fair.