Concept:Find the word opposite in meaning to "start" that also fits the context of ending a riot.Explanation:To "start" a riot means to begin it.The sentence needs a word meaning to stop or bring it under control."Quell" means to put an end to a disturbance or riot, usually by force.Therefore, "quell" is the most suitable opposite of "start" and correctly fills the gap.Answer:D. quell