This question is asking us if J is Thursday afternoon, how many of the other assistants do we know which place they go.
We know that O must be after J, so O must be on friday. Since J and K must be on the same day, we know that they have to go on Monday, although we do not know who was in the morning or afternoon. Additionally, since N must lead an afternoon session, the only open afternoon session if friday. So N must be Friday afternoon, O must be Friday morning, and O (the only one left) must fill the last open space, on Thursday morning. Since we don't know which of J/K is in the morning or afternoon, we know where three of them go.