We can get a great local diagram here once we know that O and P are in separate classes. This gives us a long chain.
(O, PH N/S)
N/S are always first as the only males who can go before K, and one of whom must go before K. K is next in the sequence. IL must go after K, as K is the first female student. G must go after IL. Finally, we know that O and the group with P are both after G. This uses all six slots. The only uncertainty in this set up is if N/S is first, as well as the order of the O and PH N/S group in 5 and 6.
Our question asks us what could be true, so we are looking for something possible. Usually this means that the answer choice will focus on those uncertainties described above, but it could be an answer that we know must be true as well.
Let's look at the answer choices.
Answer choice (A): G cannot be 5th because O and P, in different classes, must both be after her.
Answer choice (B): H cannot be 3rd, because we need H to fill the class with P in either slot 5 or 6.
Answer choice (C): I cannot be 4th, because there wouldn't be room for G, O and P to all be in separate classes after her.
Answer choice (D): N COULD be 5th here. If S is in first, N could be with P and H in slot 5 after G. This is our correct answer, and as expected, focuses on the uncertainty present in the chain.
Answer choice (E): S cannot attend the second class, because we only have 4 solo classes. The group classes are IL and P and two other students. if S is class 2, N would have to be class 1. K is a solo class, G is a solo class, and O is a solo class (in this local question at least. N, G, K, O, and S would be 5 solo classes, which is too many to have here.
Hope that helps