Pyrophobia and arachnophobia are each fears of a specific stimulus. Pyrophobia (from the Greek "piras") is the fear of fire, while arachnophobia (from the Greek "arachni") is the fear of spiders. Because they are fears of a specific stimulus, they are specific phobias as opposed to social phobias.

Post-traumatic stress disorder develops after a person is exposed to one or more traumatic events, including war, injury, assault, and natural disasters. PTSD is characterized by recurring flashbacks of the traumatic event, nightmares, hyperarousal, and high levels of anxiety.

Trichotillomania is compulsive hair-pulling, including eyebrows and eyelashes. It is classified as an impulse-control disorder, not an anxiety disorder.

A specific phobia is a fear that is directly focused on some particular trigger, object, or situation. For instance, someone who has a phobia of being the dark will always experience a heightened sense of fear whenever they are in the dark. This is much different from generalizated anxiety, where the triggers for this kind of fear are unclear and the anxiety occurs more generally.

Anxiety disorders are characterized by an unreasonable fear or anxiety. Specific phobias, a category of anxiety disorder, are a persistent and irrational fear of a specific object, activity, or situation.

Obsessions are recurrent thoughts, images, or impulses, while compulsions are recurrent, irresistible actions. A somatoform disorder occurs when there is no identifiable physical cause to explain physical symptoms. Dissociative disorders involve a splitting apart of past experiences from present memory or consciousness. Impulse control disorders are an inability to resist an impulse to perform an action that is harmful. 

Attention deficit hyperactivity disorder (ADHD) is not specifically an anxiety disorder; it involves hyperactive, inattentive, and impulsive symptomology. The other answer choices are all types of diagnosable anxiety disorders.

Agoraphobia is the fear of open spaces . People who have this condition often want to stay within the closed space of their house. This phobia is often associated with panic disorder.

Claustrophobia is the fear of closed spaces, acrophobia is the fear of heights, arachnophobia is the fear of spiders, and mysophobia is the fear of germs. 

Generalized anxiety disorder (GAD) is a mental disorder characterized by excessive worrying. Its symptoms include unproductive worrying, trouble stopping the worry process, worry about every part of daily life, and muscle tension and fatigue.

Flashbacks are one of the most prevalent symptoms of a different disorder—PTSD, or post-traumatic stress disorder. 

Although many children and adolescents may suffer from generalized anxiety disorder (GAD), it is generally not diagnosed until later in development. The other four choices are more frequently diagnosed in those under 18 years old.

Separation anxiety disorder is most common in young children, and is characterized by a consistent refusal to do things that require separation from loved ones. A common symptom is school refusal, which requires an intervention to be designed by a school-based mental health clinician (e.g., school psychologist).