定语从句：先行词是物不定代词与关系词

新概念第二册Unit2 Lesson25练习题中有一道题：
He said something and I understood it. He said something ________I understood.
（a）who
（b）whose
（c）whom
（d）which
个人理解这应该是个定语从句,something是先行词，I understood是定从，关系词我选的是（d）.
但是语法书上和老师讲的时候，都说是物为不定代词，关系词用that，不用which。但是选项里却没有that。求解物为不定代词时关系词到底可不可以用which？
如果不可以，这里怎么解释；如果可以，那若以后遇见物为不定代词时，应该怎么选？which还是that?二者区别为何？
求赐教。

需要提醒的是，anything, something等后面的形容词从句通常都用that引导，但偶尔也可用which替代that.
如：We have proved that anything which exists is real./ If you have something to say which is on the subject, wait till you have a chance and say it to the whole class. 所以选D是对的。