Necessary, because if it were not true — if a character could be truly precognitive even if NOT ALL of their perceptions of future events are accurate — then the mere fact that SOME of the perceptions were not accurate for these characters would not guarantee that those characters are not truly precognitive.

Not necessary, because even if it were possible for someone to perceive future events accurately with absolute consistency, we know from a premise that the characters in these fictional works do not perceive accurately with absolute consistency.

Even if (C) were not true and the fictional works always specify whether the future events come to pass, that doesn’t change the fact that we know there are at least some events that don’t come to pass. Perhaps the fictional works specify that for every future event, none of them came to pass? What matters is not the fact that the works specify the outcome of every event, but whether showing that some future events don’t come to pass proves a character is not truly precognitive.

Not necessary, because the argument doesn’t assume anything about what’s “central” to the plots. We know that the plots involve some perceptions that don’t come to pass. And we know some works involve characters that are supposedly precognitive. Whether these features are central to the plot doesn’t change the fact they’re in the works.

Not necessary, because the argument concerns only “many fictional works that have characters who are supposedly precognitive.” The author doesn’t have to have an opinion about other works outside of these.