答案:[解析]考查定语从句的引导词,首先what 排除。句中的先行词为(on the) trousers,定语从句为she had wiped her hands。因此表达时间的关系副词when亦排除。that为关系代词,即在从句中代替trousers行使主语或宾语功能,但在从句中可以看出she had wiped her hands.句子结构完整,因此that 排除。where为关系副词,在此代替on the trousers, 即:She had wiped her hands on the trousers.