which is the room [ ]he stayed in last week

which is the room 这是主系表结构.不在缺少句子成分.
所以后面肯定是定语从句,修饰前面的room.
排除when,因为定语从句已经有时间last week了.
排除where,因为定语从句由介词in,in不能与where同时出现在定语从句中.
that和which都可以作宾语,所以都可以.但既然特指the room,所以倾向于选择that.
that做in的宾语.