(B) and (D) are somewhat opposites in their answers so it is important to go back and understand the stimulus first. I will see if I can try and explain the stimulus better.
When choosing a candidate, Kay uses the criteria of "important" issues. In this particular case, there is only one important issue to Kay. The principle that the stimulus states is about a world where there is no single candidate that agrees 100% on Kay's important issues. In this world, it is acceptable to then pick a candidate where Kay disagrees with less on an important issue than others. Therefore, the only scenarios where it is unacceptable to vote for any candidates is when she disagrees with everyone equally or there is another candidate where she disagrees with less.
In this particular case, Medina disagrees with her on 0 issues (the optimal scenario for Kay), Legrand and Nortan disagrees with her on 1 issue. Therefore, we know that from the principle stated in the stimulus, it is unacceptable to vote for Legrand and Nortan. However, it
is acceptable to vote for Medina, that's why (D) is wrong.
Does that clarify between (B) and (D)?