参考答案：letter

解析：[听力原文]When did tile customs officers believe the man’s story 题目询问海关人员何时才相信这位男士的故事。从关键句“It was only when the man showed them the letter from the circus man that they believed him.”(直到这位男士拿出马戏团的人写给他的信时，海关人员才相信他)可知答案为“letter”。