Here we have to provide the identity of a set of orbitals that exist in the fourth principle level and F sublevel. All right, So they're telling us that it exists in the fourth principle level, which means it's connected to our principal quantum number N. Since it says 4th, that means N is 4.
And then they're giving us the sub level letter which is F which would mean that we're dealing with a 4F set of orbitals. That would mean that option C would have to be our correct answer.