NOT A TEACHER:
1. mustn't means also I'm not allowed ---> in this case the subject has to see his brother, but still he has free rein.
If the subject says I won't , he implies he doesn't want to stay ---> again, not true
2. You always have to choose the most suitable answer. A,B,C are all Ok, BUT as far as the context is concerned, B is the right fit
Student or Learner