Question

what possible reason for the ghosts appearance are dicussed?

Answer 1

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

Answer 2

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.