Which of the following is a sign of pain in animals?

Vocalization is a significant sign of pain in animals, as many pets express discomfort or distress through sounds such as whining, barking, or meowing. This kind of vocalization can be an important indicator for veterinarians and pet owners to recognize that an animal may be in pain, as animals often cannot verbally communicate their feelings in other ways. It serves as a signal that the animal is experiencing an issue that may require medical attention or intervention.

In contrast, increased playfulness, weight gain, and increased appetite are generally not associated with pain in animals. Increased playfulness might indicate that the animal is feeling well and is energetic, weight gain could be due to overfeeding or a lack of exercise rather than pain, and increased appetite often suggests that the animal is healthy. Therefore, vocalization stands out as a clear and direct sign of pain, making it the correct choice in this context.