She'll never forget her stay there______ she found her son who had gone missing two

翻译：她将永难忘记她在那儿的停留,（因为）在那次停留期间她找到了两年前失踪的儿子.
此题考查定语从句的关系词的选择.
1.先行词是her stay,而不是there,因为定语从句所修饰的先行词只能是代词/名词（短语）,而不能是副词.
此题中的stay是一个名词,意思也是“停留/逗留”.
2.将先行词her stay放入定语从句中,连接为：
(when) she found her son who had gone missing two years before
= (in/during which) she found her son who had gone missing two years before
= (in/during her stay) she found her son who had gone missing two years before
在她（在那儿的）逗留期间,她找到了两年前失踪的儿子.
很明显,该定语从句缺少时间状语.
即所谓的关系副词when = 介词(in/during) + 关系代词which = 介词(in/during) + 先行词(her stay).
3.而关系代词that/which只能等于先行词her stay,无法直接放入定语从句中做成分；
where在定语从句中充当地点状语,而不是时间状语.