Question

That is_______I can tell you . A what that B all what C all that D all which

Answer 1

C
all作前一句be的表语,that I can tell you作为all的定语从句.
由于先行词是all,所以只能由that引导,不可以用which,所以不能选D.
因为定语从句不能由what引导,所以不能选B.
如果想用what,可以把这句话变成一个表语从句,即：That is what I can tell you.选项A中的that就多余,所以不能选A.