(b) That's wrong. You should be able to recall the difference between initial impossibility, supervening impossibility, absolute impossibility and relative impossibility. Using these concepts, you can work out that the facts of the case study describe a situation of supervening impossibility (the circumstances have changed only after Sunny and Tyler have completed their contract). The facts also indicate relative impossibility, because the contract can still be performed, although in a way that is somewhat different from what was envisaged at the time of contracting.
A contract can be treated as frustrated by supervening impossibility that is relative rather than absolute, but only if the changed circumstances make performance fundamentally different from what was envisaged at the time of contracting, and only if it can be inferred from the known facts that the parties did not take the risk of changed circumstances.
See Codelfa Construction Pty Ltd v State Rail Authority of New South Wales.
The given facts indicate that, without Georgie Ripper, the production of Galactica will be very different from what was originally envisaged. It is being considerably scaled down, to an extent that will impact on the success of the show. This is sufficient to establish relative impossibility, and answer (a) can be discarded. However, the facts also indicate that, when contracting, Sunny was aware that Georgie had not yet signed, and that he might not sign: this is why she asked Tyler about this. His answer indicated that the matter was not yet settled. From this it can be inferred that Sunny and Tyler took the risk of the possibility that Georgie might not take part, and accordingly, the contract will not be treated as frustrated.