Final answer:
Frank is most likely experiencing panic disorder, which is characterized by recurring panic attacks and a fear of having an attack in public.
Step-by-step explanation:
Frank is most likely experiencing a disorder called panic disorder. Panic disorder is characterized by recurring panic attacks, which are sudden and intense periods of fear and anxiety. These panic attacks often occur without warning and can be triggered by certain situations or may happen spontaneously. People with panic disorder often have a fear of having a panic attack in public and avoid places or situations that they believe could trigger an attack, leading to agoraphobia.