Which behavior indicates that a dog is comfortable and relaxed?

The behavior characterized by a dog lying down with its body relaxed and tail wagging loosely is a strong indicator that the dog is comfortable and relaxed. When a dog is in a calm state, lying down often signifies a lack of tension or fear, allowing the dog to unwind in their environment. The loose wagging of the tail further emphasizes this relaxed demeanor; it shows a friendly disposition and indicates that the dog feels safe in its surroundings. In contrast, a dog that is standing tall with ears perked may be alert or engaged rather than relaxed. Barking playfully at strangers can suggest excitement, but it doesn't necessarily indicate a calm state. Similarly, glaring with a stiff posture typically signifies tension or potential aggression, which is the opposite of relaxation. Therefore, option B is the most appropriate response as it reflects a dog's comfort and ease.