Bear in mind that all of the answers in this question begin with the "If....." construction. So the first clause of each answer is essentially adding an additional piece of information to what is already in the stimulus. It's almost like saying "If all of the information in the stimulus is true, AND we add this extra piece of information, then is what follows necessarily true?"
In answer (D) we accept the truth of this "if" clause: that she disagrees with each of the candidates on exactly three issues important to her. So, if that is true and the stimulus is true, is it unacceptable for her to vote for any candidate?
Since we know there is at least one issue important to Kay here, the principle in the stimulus applies. That principle can be diagrammed as:
Principle:
Acceptable to support candidate with at least one disagreement
Disagree with all other candidates on more issues
Once this principle kicks in, the only way it is acceptable for her to support any candidate that differs from her on at least one issue is that she must disagree with the other candidates on
more issues than the candidate she supports.But answer (D) says she disagrees with all of the candidates on
exactly three issues. So since she can't disagree with any of the candidates more or less than the others, then it is not acceptable for her to support any of the candidates. Thus, (D) must be true.
Hope that helps!