Which of these is an example of a mental disorder?

Anxiety is classified as a mental disorder because it involves pervasive feelings of worry, fear, or apprehension that can significantly impact a person’s daily functioning and quality of life. Anxiety disorders encompass a range of conditions, including generalized anxiety disorder, panic disorder, and social anxiety disorder, which are characterized by excessive anxiety or fear responses that may manifest physically and psychologically.

In contrast, chronic pain conditions primarily relate to physical health issues, often involving the nervous system's response to injury or illness. Allergic reactions are immune responses to specific triggers and don't fall under the category of mental disorders. Insomnia, while it can be related to mental health, is categorized more specifically as a sleep disorder rather than a mental disorder on its own. Therefore, anxiety is the most fitting example of a mental disorder within the provided options.