Who was the United States freed slave involved in planning an insurrection in South Carolina?

Denmark Vesey was a formerly enslaved African American who became a prominent figure in the early 19th century and is best known for his role in planning a major slave insurrection in South Carolina in 1822. Vesey had purchased his own freedom and was an eloquent speaker, which allowed him to gather support among the enslaved population for his planned uprising against the institution of slavery. His conspiracy aimed to overthrow the system in Charleston, and although it was never executed due to betrayals, it sparked significant fear among slaveholders and led to a crackdown on the enslaved community.

The other individuals mentioned in the choices were influential in different contexts. Nat Turner led a significant slave rebellion in Virginia in 1831, which occurred later than Vesey's planned insurrection. Frederick Douglass was a key abolitionist and an advocate for civil rights, but he was not involved in a planned insurrection. W.E.B. Du Bois was an important figure in the early 20th century advocating for civil rights and African American equality but worked in a different era entirely. Thus, Denmark Vesey stands out as the key figure involved in the specific planning of an insurrection in South Carolina, making him the correct answer.