The conclusion in this argument is that the belief is mistaken. Whenever an author (or at least almost always) says that something is mistaken, this is likely the conclusion, as it requires support.
Answer choice D states this exactly.
On the other hand, answer choice B is something that the author is arguing against. For this reason, it cannot be the author's main conclusion.
I hope this helps. Please let me know if you have any other questions or would like me to elaborate further.