The passage that best supports the idea about the decline of Sparta and Athens due to their treatment of conquered peoples is the one that discusses their fall after rejecting those they had conquered as aliens.
The passage that best supports the correct answer to the previous question about the ruin of Sparta and Athens is passage D: "What was the ruin of Sparta and Athens, but this, that mighty as they were in war, they rejected as aliens those whom they had conquered?" This choice addresses the concept of inclusivity and citizenship, which is a central theme when discussing the decline of these ancient city-states. The passage compares the actions of Sparta and Athens to that of Rome under Romulus, highlighting the strategic integration of conquered peoples as citizens rather than treating them as aliens, which could lead to societal decline.