A: Plan C does not have to be on the table for someone with the Crown in play to use the Crown as their turn action to make you discard it. It is their turn action, though, not a reaction to your play (either placing it or returning it to your hand.)

Note that the Monarch cannot go on ‘fishing expeditions,’ and should only call out Plan C if it’s already been played once and its whereabouts are known. If, for some reason, they command someone to discard Plan C, and that person doesn’t have it, they must take some other action, and may not simply try another person.