Which behavior is a characteristic of an aggressive dog?

An aggressive dog often exhibits specific physical signs that indicate its readiness to confront or threaten. One prominent behavior that is characteristic of aggression is curling the lips into a snarl. This display is a warning signal, demonstrating that the dog is feeling threatened or defensive. It is an instinctive behavior meant to show teeth, which serves as a deterrent to perceived threats; this can escalate to more serious aggressive actions if the situation does not change.

In contrast, a wagging tail is typically associated with friendly or excited behavior rather than aggression. Similarly, lying down with the belly exposed usually indicates submission or a relaxed state, which is the opposite of an aggressive demeanor. Playful barking tends to suggest happiness or excitement rather than aggression. Thus, the curled lips into a snarl distinctly represents a warning and an aggressive stance in the behavior of dogs, making it the correct answer in this context.