What three reasons does Horatio list for why the ghost of King Hamlet may have returned from the grave?

This question is in reference to lines 130 and 139 in Act I, Scene 1, when Horatio, on watch with Marcellus and Barnardo, implores the ghost of King Hamlet to tell its business in the realm of man:

“If there be any good thing to be doneThat may to thee do ease and grace to me,Speak to me.If thou art privy to thy country’s fate,Which happily foreknowing may avoid,O, speak!...

This question is in reference to lines 130 and 139 in Act I, Scene 1, when Horatio, on watch with Marcellus and Barnardo, implores the ghost of King Hamlet to tell its business in the realm of man:

“If there be any good thing to be done
That may to thee do ease and grace to me,
Speak to me.
If thou art privy to thy country’s fate,
Which happily foreknowing may avoid,
O, speak!
Or if thou hast uphoarded in thy life
Extorted treasure in the womb of earth,
For which, they say, you spirits oft walk in death,
Speak of it.  Stay and speak.”

Here Horatio outlines three possibilities (none of which turn out to be quite correct) as to the ghost’s appearance, and begs the late king to give answer.  First, he asks if the ghost has any unfinished business, any “good thing to be done” to put his spirit at rest.  Second, he wonders if perhaps the ghost has information about the fate of Denmark, which, by imparting it to the watchmen, may prevent some tragedy.  Finally, Horatio asks if the late King Hamlet has some ill-gotten treasure buried somewhere, which was a common reason for spirits to walk the earth in the folklore of the day.

These speculations are all in keeping with traditional ghost lore.  There is, however, some additional context that should be mentioned in reference to Horatio’s second suggestion, outlined in lines 79-107 in his explanation for why the men are keeping watch in the first place – King Hamlet had won lands in a duel with Fortinbras, ruler of Norway, and now the son of the slain Norwegian king is raiding, and otherwise causing trouble, along their borders with Denmark to win back the claims.  So we know that Denmark is on the verge of armed conflict.  In this prior speech Horatio also makes mention of the omens foreshadowing the fall of Julius Caesar in Rome, speaking of “tenantless” graves and a moon “sick almost to doomsday with eclipse.”  Therefore he thinks that perhaps this ghost of the king could be a similar harbinger of disaster in this time of unrest.