When a group is unable to reach a consensus, group members are often accused of being stubborn, bull-headed, or unyielding.
Conclusion
If you're mad at a fellow group member because the group can't agree on something, and you want to call them some label that will stick, you should use "unyielding".
Evidence
It'll be pretty undeniable that this person has not yielded on this issue, so it will be undeniable that they are "unyielding" on this issue.
Evaluate
What weird advice for an LSAT argument to offer: "The most enduring and provable insult is unyielding!"
On Method questions, we're trying to characterize the type of evidence or type of journey we took to get to the conclusion, and the correct answer almost always involves one of these concepts: Analogy Implications of Logic Rule Out Alternatives Call Out Dubious Assumption Raise Overlooked Information (Counter)example to Principle Make a Distinction Define a Term Alternate Explanation
This argument seems to involve some "ruling out alternatives", since the author is suggesting that we do not go with "stubborn or bull-headed".
Goal
Look for an answer possibly saying that the author endorsed one option over a couple others by distinguishing it from the other two in terms of its undeniable truth.