To determine the best statistic for posting advertisements for a job fair, the most relevant choice would be **unemployment rates**.
Here's the reasoning behind this choice:
- **Unemployment Rates:** This statistic provides insights into how many people are actively seeking work but are unable to find it. High unemployment rates in an area may indicate a larger pool of potential job seekers who would likely be interested in attending a job fair to explore employment opportunities.
- **Population:** While the total population can provide some context, it doesn't directly indicate how many people are looking for jobs.
- **Education:** Education levels can help understand the types of jobs that may appeal to candidates but do not directly show the demand for job opportunities.
- **Income:** Income levels might indicate the economic status of individuals; however, it does not directly imply how many people are unemployed and seeking work.
In summary, **unemployment rates** are the best statistic to assess where to post advertisements for a job fair, as they directly reflect the job-seeking population in an area.