The publication of the "Crack-Up" essays looks now like (i) ___, marking a (ii) ___ in American consciousness and therefore in narrative voice, an evident moment when the center of authorial gravity shifted from the alleged (iii) ____ afforded by fiction‘s third person to the presumption of greater authenticity provided by the first-person voice with all its limitations.

Lets try to paraphrase it
"Crack-Up" essays now look like ___, marking a ___ in minds and narrative voice of Americans, which can be clearly seen by the shift of writing tone from an alleged ____ to something which is presumably more authentic with its own limitations.

Blank(iii) - there is a clear indication of a -ve word, we can guess the nature to be not-authentic, probably pompous, ostentatious. Now lets look at the options:
"ingenuity" and "sincerity" are not words we are looking for, clearing making for "omniscience" - meaning all knowing, could ring a tone of being supercilious without even bothering to back up with explanation.

Blank (ii) - it is clear from the context is that there is some form of "shift" happening, however what is not clear or is not backed up enough is "reinvention" and "denunciation", thus "fundamental shift" fits perfectly

Blank (i) - the thing about such blanks is (as is the case ii as well) - all options seem to fit in a general sense, especially if you look at it without the context or with wherewithal of keywords from the sentence. Lets go through the options and check do we have sufficient context of keywords to back it up with.
a. overwhelming feat - do we know is it some kind of achievement which is mindblowing -> no, there is no such mention, not even any kind of appreciation, praise or any form of approbation
b. an abrupt reversal - "abrupt" is still something we can try to make peace with, however for "reversal" we dont have any info saying initially we were in a state which now got reverse
c. a sharp pivot - "pivot" yes, what we know there is a change (note - not a reversal but a change)