She rode a car at-----I think is a dangerous speed.

应该填 what
如果用which,就变成了一个定语从句,修饰前面的名字,但前面是at,所以不能是which. 如果句子变成She rode a car a speed ____ I think is dangerous, 则该选which.
拿不定注意的话,自己上网验证一下.上google English搜索“at what I think”和“at which I think”,从搜索结果你就可以知道哪个是正确的了.