Which of the Following Examples Would Be a Hallucination

Which of the following examples would be a hallucination?

A) A man believes that his dead wife is talking to him.
B) A woman hears the doorbell ring and goes to answer it,but no one is there.
C) A child sees a man standing in his closet.When the lights are turned on it is only a dry cleaning bag.
D) A man believes that the dog has curled up on the bed,but when he gets closer he sees that it is a blanket.