Answer:
To be honest, due to this being a English question about slavery. It's not my best subject, though the only answer I can give you would probably be the second choice, 'to show that slavery is harmful to society'.
Step-by-step explanation:
It's the only reasonable one. The last one is proving that slavery is right. The first one is just not as reasonable, and I just have a feeling it's the second one. Though, it might be wrong it's most reasonable in my case. Good luck!