可以这样理解,从句意思是he truned a deaf ear to which(指the warning I gave him),意思是我给他警告他不听,turn a deaf ear to sth,不听某事,所以用to which.
这句话如果这样表达也是可以的：I gave him a warning,which he turned a deaf ear to.原句只是把介词to提前
例：Here are the top ten highest paying occupations for which you don't need a degree
也可以表达成：
Here are the top ten highest paying occupations which you don't need a degree for
将for提前与否不要紧,只要它仍是从句形式.
第二句,这是这么严重的一个错误,我认为它是粗心造成的.定语从句,which一般要放在从句首,但不排除像第一句那样将介词提前的情况,那样也是可以的,但像第二句的B选项绝对是错误的,如果选B,那还是从句吗?I think which was caused by carelessness,主谓宾,完完整整的一个完整句,显然不能做从句,所以which一定要提前才是定语从句,或者将逗号改为句号,选B就可以了,那样的话就是两个句子表达同样从句一句表达的意思.
当然是充当要修饰的对象,引导名词性从句,可以修饰主语或宾语