你好! which只能以疑问代词的身份(即意思为“哪个”)来引导表语从句,如: The question is which he will choose. 若以关系代词的身份,则只能引导定语从句(一般可以用that代替,或者省略),如: This is the question which/that I want to ask.
题目句子的空格处,显然不能解释为“哪个”,所以不能用which. 再举个相似的例子: This is what I want to ask. ——不能说 This is which I want to ask.