Your answer certainly is not a sufficient assumption b/c it doesn't prove the conclusion 100% valid. Change the 'never' to something less strong and it could be a necessary assumption.
Remeber, there are numerous (possibly an infinite amount if you consider defenders) necessary assumptions for every argument. Do not rule out one in an answer choice just b/c it is not the one that you prephrased.
Why wouldn't the answer I chose be a sufficient assumption? If that answer was a pertinent principle, then it would justify the stimulus's evaluation, wouldn't it?
(I do realize why the answer choice I chose was incorrect for this particular question stem - it is far too strong to be necessary. To tell you the truth, though, by the time I had reached the answer choices, I had forgotten this was a necessary assumption problem, which is why I chose that answer choice, thinking it was a sufficient assumption problem. I do want to know, though, if I erred in categorizing that answer choice as a sufficient assumption.)