Where are the sentences? This statement has no valid context in order to be answered
The passage that best summarizes the central idea is D. they do not rashly engage in war, unless it be either to defend themselves or their friends from any unjust aggressors, or, out of good nature.
The whole excerpt tells us that Utopians despise war but, nevertheless, train for it and engage in it in certain scenarios. Passage D is the only one that tells us both these things: that they don't like war but engage in it sometimes.
