Which type of aggression is exemplified by a dog expressing hostility?

The correct choice is that a dog's expression of hostility can be encapsulated by the idea of "all of the above." Each of the specific forms of aggression mentioned contributes to a dog's overall behavior when displaying hostility.

Aggressive barking is a direct vocalization that can signal a threat or intimidation and is often a form of aggression that a dog uses when it feels threatened or wants to defend its territory. Fear-based aggression occurs when a dog reacts defensively out of fear; in these cases, the hostility is often a protective mechanism. Territorial behavior highlights a dog's instinct to protect its home or personal space from perceived intruders, leading to aggressive displays to safeguard that territory.

Thus, hostility in dogs can manifest through aggressive barking, fear-based reactions, and territorial defense, all of which fall under the broader umbrella of aggressive behavior.