This is Must Be True question regarding "slavery" part of the passage.
Let's go to the answer choice straight away.
A) Whether they were prepared or not was not given. We are only given that rebellions were common but less collective.
B) Few of them owned MORE than hundreds worked, 2 percent to be exact. (25-28)
C) Unsupported. Management styles were different with the existence of intermediaries, but which was efficient was not dealt here.
D) This is it. Paraphrase of 32-34 Correct
E) While size of serf and slaves were dealt with, counterbalancing with how common those were makes it uncertain to find out what the average is. Thus, we can't infer the accurate fact about this, nor are we given information regarding this.