Based on the chart provided, the correct response to how westward expansion affected slavery in the United States between 1820 and 1850 is:
The enslaved population roughly doubled as several new slave states were admitted to the Union.
This conclusion is supported by the data: the number of enslaved people increased significantly from 1,539,038 in 1820 to 3,204,313 in 1850, indicating a substantial rise in the enslaved population during that period.