In reading the first two sentences, I assumed they were setting up a false choice:
if you compare to people above you ---> _____
if you compare to people below you ---> _____
If the conclusion had made that seem like an exhaustive set of considerations, we'd retaliate with, "What about comparing yourself to people at your level?"
Instead, it's saying
If you don't compare to people above/below/equal you ---> _____
So the conclusion is really guilty of trying to do an illegal negation. It's not full-on conditional language (almost invariably / almost invariably / most likely be), so the answer would probably not use Nec and Suff, but it would sound something like,
"takes for granted that the absence of conditions that typically lead to certain effects would make it likely for those effects to be absent"
I would also think through Anti-Conclusion the way you just said, and look for weakening ideas that allow me to argue, "EVEN IF we avoid comparing ourselves to others, WE MIGHT STILL BE likely to be self-disparaging and dismissive."
When I see (C), it seems like treating it as a Necessary Assumption, then negating that conditional, would be a complex route to go.
In general, when I see conditionals in Necessary Assumption answers (including "takes for granted / presumes" answers on Flaw), I don't ever try negating it.
I just ask myself, "does this conditional statement represent a move the author made?"
Did the author ever go from
IF you're dismissive AND self-disparaging --> _____ ?
No. She had these three moves:
IF compare up --> self-disparaging
IF compare down --> dismissive
IF don't compare --> self-accepting and accepting of others
(C) is just trying to pretend like the author made some conditional out of all those consequences.
(C) is actually saying something self-justifying.
Self-accepting is the logical opposite of self-disparaging.
Accepting of others is the logical opposite of dismissing others.
So, yes (C), if we are strong and tall, then we are not weak and short.
If we are rich and attractive, then we are not poor and ugly.
But this would never be considered a Flaw, since it's just internally true.
Remember that for all Flaw answer choices you can ask yourself:
1. Is this true?
2. Is this a problem with the logic?
(C) is true, but it's not a problem with the logic. The logic is all about how we arrive at or evaluate the conclusion, and (C) has nothing to do with getting us to the conclusion or evaluating the conclusion.
Hope this helps.