The correct answer is - By having the same actor play both parts, the two characters become one and Hamlet's madness is confirmed.
When both Hamlet and the ghost are played by one character, we realize that the ghost never existed in the first place and that it was only a figment of Hamlet's imagination. When there is only one actor, we see that it is all in his head and that he is slowly going mad because of the death of his father.
When played by the same actor, the adaptation gives off the impression that the ghost is actually a figment of Hamlet's imagination, that is, that it's all in his head. When there are two actors, then Hamlet's motivation does not symbolize inner quarrels.
