The correct answer is D) Conflicts over slavery in the United States resulted in violence on a nationwide scale.
The factor that slowed the progress of the abolition movement in the United States was "Conflicts over slavery in the United States resulted in violence on a nationwide scale."
Truly speaking, the nations started to divide more and more in those years because of the issue of slavery. Northerners heavily supported abolitionism while southerners were in favor of slaves because they depended so much on slaves to produce the kinds of crops they need to grow in the large southern plantations.
What helped spark a major abolitionist movement in the 1820s was the Second Great Awakening.
The beginning of the 1800s represented a moment in the history of the United States where the Protestant religious movement lived a moment of expansion that some historians called "revival." It was the Second Great Awakening that started approximately in 1790 and ended in 1840.