选which room he stayed in
因为后面有个介词in ,如 in the room
where 一般不与介词搭配的请举例比如，你要去哪里， where are you going? 如果要用which来提问的话，只问，你要去哪个地方？ 即which city are you going to ? where 是副词，一般不会与介词联用的。