What possible reason for the ghosts appearance are dicussed?

That he has come to ask Hamlet to avenge his death.
That he is a demon taking the shape of Hamlets dad to trick him into committing evil

Most people say that ghosts are real some say they are not but in reality no one really know because half the time it's a hoax but i wouldn't call them ghosts i call them trapped spirits still looking for something.

Number 2 the key is a bleak reminder