I see the appeal of answer choice D, it is something that was mentioned specifically in the passage. However, we will often narrow it down to a couple of answer choices, only to find that neither of them has an obvious error. At that point, we have to pay attention to what details they are missing, and ask ourselves if they fit the tone of the author's argument. One of them simply won't express the main point as well as the other. This is a really good example to practice with.
What is the point of saying that modern criticisms cannot negate a patient's need for assurance? It is true, but it is not the author's main point. Rather, it is support for the author's conclusion that the fundamental elements of the Hippocratic oath should be retained. In short, answer choice D is meant to support answer choice B. Similar to some logical reasoning questions, it is a sub-conclusion leading to a main conclusion. Support for B and D can both be found in the same block of text (lines 42-52). Take a closer look, and you'll be able to tell which is the main conclusion.
This is a tough question, so don't beat yourself up. If you learn to apply this level of analysis to reading comprehension passages, you will be just fine.
