How did Horatio answer Marcellus' question in "Hamlet"?
I am guessing that the question you refer to is in Act 1, sc. 1 when Marcellus asks, "Who is't can inform me?" asking about why the country is making war preparations and the guards have been increased. Horatio says he can give Marcellus an answer, or at least the answer as far as he's heard. He says that young Fortinbras has been gathering an army of mercenaries to take back the land his father lost in a battle with King Hamlet. Fortinbras' father had lost these lands as part of a valid, sealed agreement stating that whoever lost the battle would turn over his lands to the winner. King Hamlet won and Fortinbras' father lost. Denmark is preparing for the possibility that Fortinbras will attack. This sets the stage for the political undercurrent that runs through the play and for Hamlet's vow in Act 4, sc. 4 that he will concentrate solely on avenging his father's death from that point on and for the eventual claim of the throne by Fortinbras at the end of the play.
In ActI sc1 on two earlier occasions Bernardo and Marcellus, guards in the palace of the King of Denmark, at the stroke of one A.M. had seen the ghost of the recently deceased King of Denmark. They had informed Horatio the friend of Hamlet the Prince of Denmark, but Horatio being a rationalist refused to believe Bernardo and Marcellus. So, tonight he has come to see for himself and investigate the accuracy of Bernardo's and Marcellus' story. The ghost doesn't disappoint them, at the stroke of 1 A.M. it appears. All of them are terrified and when Horatio questions it, it withdraws.
Horatio is now convinced of the existence of the ghost and he remarks that the ghost is dressed just like when the King of Denmark fought with the King of Norway and the Poles and concludes that the appearance of the ghost is a bad omen.
Marcellus at once asks him why such hectic preparations are going on in Denmark to get the entire nation ready for a major war. Horatio replies that the King of Norway once challenged the King of Denmark in battle and was slain by him. Consequently according to the legally binding agreement he also lost a lot of territory. Now, the son of the slain king of Norway wished to take back by force illegally this lost territory. So a national emergency has been declared and Denmark is getting ready to prevent this from happening.