b. "... not complaining of the past, but simply asking for a better future...."
Antithesis is when two opposites are used to show contrast in a parallel structure. Option b is the best choice because Douglass is contrasting the past with the future. He does this to show the purpose of his speech is not to look at the past but instead to look towards the future. The opposites are past and future. There are no opposites within any of the other options, so they simply will not work.