Which two factors are commonly known triggers for canine aggression?

The correct answer highlights fear and territorial invasion as commonly known triggers for canine aggression. Fear is a significant emotional response in dogs; when a dog feels threatened or scared, it may resort to aggression as a means of self-defense. This instinctual behavior is a survival mechanism, prompting the dog to act out against perceived threats.

Territorial invasion also plays a crucial role in canine aggression. Dogs are naturally territorial animals, and they can become protective of their space, belongings, or family members. When an unfamiliar individual or animal enters this space, it can provoke an aggressive response as the dog seeks to defend its territory.

To understand why the other options are not as fitting: while hunger and playfulness can influence a dog's mood and behavior, they are less likely to lead to aggression directly compared to fear and territorial instinct. Loneliness and boredom can lead to behavioral issues, but these are typically not direct triggers of aggression. Age and size may influence a dog's behavior, but they are not direct triggers for aggression in the same way that fear and territorial invasion are.