The author most likely arranges the sentences in this way C. to show that early U.S. presidents acknowledged the evils of slavery even while participating in the system.
The sequence introduces James Madison and his condemnation of slavery, then presents Thomas Jefferson's acknowledgment of the horrors of slavery, highlighting that both were influential figures in America's founding while also being complicit in the institution of slavery themselves. This arrangement emphasizes the contradiction between their roles as founders advocating for liberty and their participation in slavery.