I strongly believe that it is B. I know it isn't D because it doesn't compare or contrast asthma to something else.
I think your answer is A. I hope I helped!
It sounds like Fredrick Douglass was pointing out that slave masters didn't want their slaves to have knowledge such as reading and writing, let alone knowing their own ages and such.
Most slave masters wanted this so that they could feel superior over blacks, and they didn't want the slaves to be equal with them.
I think it would be the third option because it has a significant less amount of punctuation that the others?