The argument is not trying to discredit the fact that "homelessness is a serious social problem," it is trying to discredit "further government spending to provide low income housing" as cure for the homelessness.
Thus, answer choice (D) is not correct since the statement "that homeless is a serious social problem" is not being discredited in the argument.
Check out the other two threads for this question in the message boards to see the discussion on why (C) is the correct answer.
Hope that helps! Please let us know if you have any other questions.
