With this trap Miss Marple reveals Dr. Quimper's real motive and has him arrested by the police.

trap 是名词，意思是"陷阱"，指用来捕捉或揭露某人真实意图的巧妙设计或计谋。在这个句子中，trap 比喻马普尔小姐设下的巧妙布局，用来揭露奎珀医生的真实动机。