Which of the following is most likely to increase aggression in pets?

In the context of pet behavior, stressful veterinary visits can significantly increase aggression in pets. During such visits, pets may experience anxiety due to unfamiliar sounds, smells, and the presence of other animals, which can provoke a defensive or fearful reaction. This heightened state of stress can lead to aggressive behaviors as a way for the pet to cope with fear or discomfort.

The other options contribute positively to a pet's overall well-being and may help reduce aggression. Adequate socialization typically exposes pets to various environments, people, and other animals, helping them feel more comfortable and less likely to react aggressively. Positive reinforcement training fosters a trusting relationship between the pet and the owner, promoting good behavior through rewards rather than intimidation. Consistent routines also provide pets with a sense of security, reducing anxiety and the likelihood of aggressive responses.