After dropping $1200...
I'm getting sick of Princeton Review bragging about having all these annotations online. How do they explain the correct answer? "(C) This is the right answer, as it is consistent with what the author said."

They don't elaborate anymore than this and the instructor just rehashes the online annotations. They do a decent job of summarizing the paragraph, but I can do that, I need to better understand what makes one answer choice better than the other.
It's a little arbitrary.
Preptest 24 - Section 2 - Question 9:
Original Question:
Historian: Anyone who thinks that the terrors of the ancient regime of Q were exclusively the work of fanatics is overlooking a basic truth: the regime was made up primarily of ordinary people enthusiastically seeking paradise. The regime executed many people in the pursuit of its goal; but it later became clear that paradise, as they defined it, is unrealizable. So at least some of the ordinary people of Q were in fact murderers.
Which one of the following principles, if valid, provides the most support for the historians argument:
[The two choices I narrow down to:]
(A) The pursuit of paradise does not justify murder
(C) Execution in pursuit of what is later found to be unattainable consitutes murder
Their annotation:
Question 9: This is a principle question. You have to support the Historian’s argumentation. He argues that the regime of Q was made up of many ordinary people and because they did not achieve paradise, many ordinary people were murderers. You must now justify this reasoning.
(A) Whether the murders were justified is not addressed in the argument.
(B) Whether the pursuit of paradise is justified is also not addressed in the argument.
(C) This is the right answer, as it is consistent with what the author said.
(D) Inhumanity is not relevant to the argument since it is never mentioned.
(E) Fanaticism is never defined in the argument.