Enlightenment thinkers often justified continuing slavery primarily by the belief that they did not count enslaved people as fully human or as individuals with rights. This dehumanization allowed some to rationalize the institution of slavery as acceptable and necessary for economic and social structures. While there were various arguments and rationalizations, the most prominent one was the perception of enslaved individuals as less than fully human.
So, the best response from the options provided is:
They did not count enslaved people as people.