The correct answer to this open question is the following.
Although there are no options attached we can say the following.
The problem with the southern plantation owners' claims that enslaved African Americans had a better life than African Americans working in the North was based on the fact that in the North, people were abolitionists and were against the institution of slavery.
In the South, wealthy landlords had slaves and forced them to long for long hours under the sun producing cash crops that had to be traded and exported to Europe. Black slaves lived a difficult life in the South, and that is why they wanted to escape to the North to be free.
Let's put the example of the Underground Railroad.
The Underground Railroad operated before the Civil war. It was not a road at all, but a group of people that wanted to help slaves to other territories where slavery had been abolished. Basically were the Quakers, the group that formally helped African Americans to leave the slavery states.
One figure has the spotlight when talking about helping southern slaves escape to the northern states: Harriet Tubman.
She was a supporter of liberty and always wanted to help black slaves from the south to be free. That is why she escaped from slavery and later help many of them to get to the North before the beginning of the American Civil War.