I don't know----I can do for you.A.who B.that C.what D.if为什么

选 C what,引导宾语从句,what是指代从句中的宾语的.
如果你选D的话,从句就没有宾语了.如果从句说明了do什么for you,就应该选D.比如I don't know if I can do it for you.