Why is D correct?

I think there are two approaches here. In some cases, when the author has a clear point of view that she is advancing, we can hint for that in the answer choices. In most cases, however, I use process of elimination on main point questions, knocking out answer choices that clearly are not what the passage said, or narrowly recount an ancillary part of the argument. Here, I don't think the passage exactly said C. I think the author does argue that what it means to be a physician is misunderstood, but I don't see where she thinks it's the sole cause of the efforts mentioned in the first paragraph. That said, I think the author does have a conclusion that she is arguing for: that doctors are given the respect of a professional because they, through their profession, serve a higher purpose. We see this around lines 50-60. She is structuring her argument to show this, that there really is a reason that medicine is and should be held up in the way that it is.