Many researchers have chipped away at the edges of Vera‘s argument, (i) ______ his take on the data or his interpretation of historical documents. But no one has directly (ii) ______ the heart of his argument: that we have wildly underestimated the impact that animals, especially large ones, had on the environment. Even Vera‘s critics say they appreciate the debate he has stirred up, if only because it has made them (iii) ______ their convictions.

Blank (i):
Clues are - "many researchers chipped away" at Vera's argument -> indicating some kind of "attack"
->"quibbling with"

Blank (ii):
Clue - "but" indicating opposite direction. Earlier they were attacking, it now they seem to accept some part of his argument, which is backed by the remainder of the sentence -"that we..."
Also be careful of "no one" -> basically no one argued with him on this front. So though there was a change in the flow, due to the appearance of "no one" we are looking for a word with negative connotation

indicating "confronted" ("supported" - opposite to what we need, "apprehended" meaning understood -> again opposite ...these would have made sense had there been "everyone" instead of "no one")

Blank (iii)
These arguments have initiated some kind of trigger that is making them revisit, analyze their convictions. All options are positive and very close to each other. "revivify" and "reaffirm" are quite strong words, but we dont have any indication that Vera's argument was so strong that these critics would be proselytized.
"reexamine" is the best fist - where the argument is just a trigger for them to revisit their convictions