Based on the chart and the data provided, the correct response is:
The enslaved population roughly doubled as several new slave states were admitted to the Union.
Between 1820 (1,539,038 enslaved individuals) and 1850 (3,204,313 enslaved individuals), there is a significant increase in the number of enslaved people, indicating that westward expansion led to the establishment of new slave states and an increase in the enslaved population.