She'll never forget her stay there she found her son who had gone missing two years befor

问题补充：

Shell never forget her stay there she found her son who had gone missing two years before.

A. that

B. which

C. where

D. when

答案：

A. that

B. which

C. where

(答案→)D. when

解析：先行词her stay指时间，并且在定语从句中充当状语，故用关系副词when引导定语从句。