What characteristic is indicative of a dog being aggressive?

A characteristic that is indicative of a dog being aggressive is when the hair stands up along its back and neck. This phenomenon, often referred to as "piloerection," occurs when a dog is feeling threatened, fearful, or assertive. The standing hair can serve as a visual signal to other animals or humans, indicating that the dog may be preparing to defend itself or its territory.

The other options presented do not typically correlate with aggressive behavior. For example, panting quietly can be a sign of anxiety or heat but is not a direct indicator of aggression. Playful barking usually signifies that a dog is engaged in fun, while chasing its tail is a behavior often linked to playfulness or self-entertainment, rather than aggression. Thus, the specific body language of hair standing up is a clear and significant sign of aggression.