The correct answer is C.
The central idea of this passage is that the US needs to grant its black population freedom and equality.
Douglass makes the argument that this country cannot reach its maximum potential if a good portion of the population is oppressed. He compares the oppression of African-Americans in the United States to cutting off the right arm of every soldier going to war -- senseless and dangerous.
Rounded to the nearest hundredth, 6.88 would still be 6.88
This is a complex sentence which consists of a dependent or subordinate clause and an independent clause.
'If you finish your yard work' -- dependent clause
'you can take the car to the movie' -- independent clause.