You're correct that N could be at two on thursday. In fact, it is also possible that M is at 1 on thursday. However, this question is asking what must be true, rather than what could be true. It is also possible that N is at 1 or 3 on thursday, and that M is at 2 or 3 on thursday. However, we know that O is on friday, and the J must be the first on friday and K must be last, so the only space for O is 2pm on friday. Thus, it must be true that O is at 3 on friday.
