In act i, scene ii, claudius’s mention of fortinbras raises the issue of

corruption in denmark’s government
how fortinbras is better than hamlet
the cause of king hamlet’s death
an external threat to denmark

an external threat to Denmark.

Young Fortinbras wants to reclaim the territory that was taken away from Norway by Denmark (especifically by Hamlet's father). These events led to the death of Old Fortinbras, so the young prince wants to regain the territory in an attempt to avenge his father's death. He is a determined man, and has been able to raise an army to help him in his mission. This represents a significant external threat to Denmark.

the answer to "what does the narrator realize by the end of the story" is b.

answer: librarians is the correct word.