Hey there!
The answer is A. That slavery was still allowed in a free nation.
One of the things the colonists fought for was freedom. Freedom of speech, freedom of religion, and freedom of the press along with other things.
However, for hundreds of thousands of slaves torn away from their homes, they had none of these freedoms. They were considered inferior and not people.
A paradox is a statement that seems absurd of self-contradictory, which this statement very much is. If the U.S. is a free nation, then why do hundreds of thousands of people living there have no freedoms, stolen from them by the very people who advocated for their own freedom?
Hope this helps!