(c) That's right. This question involves one of the recognised circumstances which may invalidate a legal transaction, in particular, mistake.
It is clear from the facts that this is not a case of a bilateral mistake, because Tyler was not mistaken about whether or not Georgie Ripper was already committed to taking part in the show. You were told to assume only that Sunny formed this erroneous belief. The case is one of unilateral mistake. Answer (a) is therefore wrong. The law is not inclined to treat a legal transaction as unenforceable on grounds of unilateral mistake unless very special circumstances exist.
These circumstances are laid down in Taylor v Johnson. It must be shown that, in addition to one party to the transaction being seriously mistaken, this fact must also have been obvious to the other party, and that party must have taken unconscionable advantage of the situation to complete the transaction before the error was discovered.
While Sunny was seriously mistaken, the facts do not indicate that Tyler must have been aware of that fact, and that he took unconscionable advantage of the situation. He appears to have told Sunny exactly what the situation was, and she did not say anything to make her misunderstanding obvious.