Who doesn’t respect him That vs But knows how much he has sacrificed for the country?

This is a question in an English test paper. Who doesn’t respect him ________ knows how much he has sacrificed for the country? A. that B. who C. / D. but Some think the answer is “D. but”, which to me is not logical. I think “A. that” is the right choice. Who doesn’t respect him that knows how much he has sacrificed for the country? →Who that knows how much he has sacrificed for the country…