You are right. However, not all the time the official material has a clear-cut among the answer choices.
Look this question
https://gre.myprepclub.com/forum/since-she ... html#p6323B is very very close and tempting but is wrong. it has a strong argument.
But what you are saying makes sense.
@edit
looking at the middle part of the sentence
he had no right to (ii) absolve himself. Which means that he was guilty and for a very good reason
it was his (iii) ________ that caused the debacle.
His bitterness or ill feeling (acrimony) was the only reason for the failure.
Skill and modesty are clearly out of scope.
Often a dialogue fails because we are ill-prepared or too emotional. Often skills or modesty are not implied if not in small amount.
From this
Regards