Thanks for the question! So your’ right that there’s no rule stating that F has to be on the same floor as S, and indeed, there will be answers to this game that have F and S on different floors. But remember the hypothetical the question gives us: we have to put P on the middle floor along with exactly two other departments. That severely constrains our options, and once we follow the implications of what this hypothetical means, we’ll be led to answer choice (C).
So we have to put P on the middle floor with two other departments. What does this mean about the other two floors? Well, one of them has to be labor by itself based on rule 3. And the other has to have the other three departments.
Now, what else do we know? We also know that P and T have to be on the same floor based on rule 1. So the middle floor is P T _ as far as we know right now.
But we also know that H has to be on a floor above I. So one of H or I have to be in the middle floor. So right now we have one floor with L, one floor with P T H/I, and the remaining floor, by process of elimination, must have F, S, and I/H. But that means that F and S have to be on the same floor, and that’s how we get to answer choice (C).
Hope this helps! Feel free to ask any other questions that you might have.