I’ll never forget the most wonderful holiday we stayed in the country with my grandparent

问题补充：

I’ll never forget the most wonderful holiday we stayed in the country with my grandparents.

A. when

B. which

C. where

D. that

答案：

A. when

B. which

C. where

D. that

解析：A

解析先行词为holiday，还原之后可看出when在定语从句中作时间状语。