To accurately respond to your question, I would need more context about the chart you're referring to. However, if the chart pertains to the relationship between cotton production and slavery during the 19th century in the United States, then the most logical option would likely be:
B. the population of slaves in the South increases
This choice reflects historical trends, where the demand for cotton (especially after the invention of the cotton gin) led to an increase in the slave population as plantations expanded to meet cotton production demands. If you can provide more details about the chart or its context, I can offer a more precise answer!