Concept:This sentence uses a third conditional, which talks about a past imaginary situation and its unreal result.Explanation:The clause says "If the police had not intervened quickly..."This describes an unreal past condition. The expected main clause must be written as wouldhave + past participle.The mob did not kill the suspect because the police stopped them.So the correct form is wouldhavebeenkilled.The phrase wouldhavebeenkilled shows that something prevented that bad event from happening.Answer:A. would have been killed