The reasoning in the argument is most vulnerable to criticism on the
grounds that the argument
A)_____ B)_____ C)_____
For me, it sounds like most should be eliminated in order for the sentence to be correct, since "criticism" is a singular in this sentence. If the sentence is "The reasoning in the argument is most vulnerable to which one of the criticisms?" then it would sound ok with the word most. Am I thinking incorrectly?
In this case, criticism is an uncounted noun; it refers to the act or process of criticism, rather than to a specific objection to the argument. It could also be phrased "Most vulnerable to attack" in the same way.
Most vulnerable in this context implies that the argument may be vulnerable to criticism in other ways, but this particular objection to it is the most obvious or the strongest.