Based on the chart, the correct response is:
The slave population roughly doubled as several new slave states were admitted to the Union.
Between 1820 and 1850, the numbers increased significantly (from 1,539,038 in 1820 to 3,204,313 in 1850), indicating that westward expansion and the admission of new slave states contributed to a substantial increase in the enslaved population in the United States during this period.