Final answer:
Laws related to Slavery had the greatest impact on the Deep South where slavery was more entrenched, making option B, the South, the correct answer.
Step-by-step explanation:
To determine which law had the greatest impact on the population of a specific part of the country, we must reference historical context provided by the question. When referring to laws that had a significant impact on the regional populations in the United States, particularly during the 19th century, the law discussing Slavery would be one of the most influential. Based on the information provided, we understand that Slavery was more deeply entrenched in the Deep South compared to other regions such as the Upper South, border states, or the North. This indicates that any laws or changes impacting Slavery would have the greatest effect on the South. Therefore, option B, the South, is where such a law would have had the greatest impact.