The flaw in the author's reasoning lies in his reasoning from part to whole, which is implied rather than explicitly stated: "Each of the singers has an expressive voice, therefore all the choir, as a group, must be expressive and the critic must be wrong." But just because each voice is expressive alone doesn't necessarily mean that all the voices will be expressive together. (D), therefore, is correct.
(A) Although the author is rather vehement in disputing the critic's claims, he doesn't address the critic's character.
