For Q31(c), it seems you need to assume that the four students are mutually independent with respect to their availability on next Saturday night. I don't think was able to be deduced from what is given in the question. Given that part (a) was all about independence with respect to availability next Friday and Saturday night for a particular student, you would think it's even more important than normal to state that the four students are independent from each other with respect to their availability (if that indeed was what was supposed to be assumed).
One scenario could be that all four students are best friends and always go out together (if they do go out) on Friday and/or Saturday nights. All the probabilities given earlier in the question could still apply. If that were the case, then the probability that at least one is not available next Saturday would be 1/5. This is clearly different to what I think was intended, namely 1-(4/5)^4.
Do you agree? Or is this question just fine as it is and I'm wrong?