All the characters in this novel are fictitious.

The word “unbelievable” means “difficult or impossible to believe.” This does not necessarily mean that the characters in a fictitious novel are not believable. In fact, many fictitious characters are very believable. However, they are still not real people.

The word “unreliable” means “not able to be trusted.” This does not necessarily apply to the characters in a fictitious novel. In fact, many fictitious characters are very reliable. They are consistent in their behavior and their motivations are clear.

The word “infamous” means “having a very bad reputation.” This does not necessarily apply to the characters in a fictitious novel. In fact, many fictitious characters are very well-liked. They are often the heroes or heroines of the story.

Therefore, the correct answer is D. unreal.