what只用于賓語從句中．（先行詞是動詞）
that which 可以用于定語從句中．（先行詞是名詞）
這道題應該要選C吧?
PS.
介詞+疑問代詞=疑問副詞.其中是因為從句中的介詞提前,所以有這種情況.e.g：I may have to to into hospital,in which case I won't be going on holiday.我可能得住院,如果那樣我就不去度假了.
關于地點的不知道這句行嗎：
I would like to go to the place where I can see the beach.