I don't understand where the solution for part C coming from. What I thought is also Vo/Vi. I found Vo as 303 mV and Vi as Vs*(10/10+100). However in solution it seems my thought is wrong. How should I think?
You need to take into account the circuit without the generator (and then the associated resistance). Once you detach the generator you should use a voltage divider using RO and RL to find the output voltage VO dependant on Vi (and so the requested transfer function).