To properly complete the chart, more context is needed about what the chart is indicating or representing. However, if we consider typical historical trends related to the cotton industry and slavery in the Southern United States, option C ("fewer slaves are needed to work on the plantations") could be the most logical completion.
This is because advancements in technology or changes in the economy (like a drop in cotton prices) would reduce the demand for labor, potentially leading to a decrease in the number of slaves needed on plantations.
If you have more specific information about what the chart is addressing, please provide that, and I'd be happy to help further!