Based on the chart and the data provided, the correct answer would be:
The slave population roughly doubled as several new slave states were admitted to the Union.
Between 1820 and 1850, the number of enslaved people in the United States increased significantly, from 1,539,038 in 1820 to 3,204,313 in 1850, indicating a substantial rise in the slave population, likely due to westward expansion and the admission of new slave states.