My parents used -----they had to get a new car for me .

C是正确的!all that=what. 除了that是定语从句,what引导宾语从句不同外,句子的意思没啥不同.
只能用all that或者what.不能用all what,因为what不能引导定语从句.
ALL that he did is good for us. 他所做的一切对我们都是好的.=
What he did is good for us.他做的对我们都是好的；