Final answer:
William's behavior of avoiding a place where he feels escape might be difficult if in danger is indicative of agoraphobia, not a specific phobia, generalized anxiety disorder, or panic disorder.
Step-by-step explanation:
William walking up 20 flights of stairs to avoid a situation where he feels he cannot escape if the elevator stalls indicates a behavior that is most appropriately classified as agoraphobia. Agoraphobia is an anxiety disorder that involves an intense fear and avoidance of places or situations where escape might be difficult or help might not be available in the event of a panic attack or panic-like symptoms.
This is not a specific phobia, which is a fear of a particular object or situation, nor is it generalized anxiety disorder, which involves widespread anxiety not limited to specific situations. Panic disorder would involve spontaneous panic attacks, which is not indicated by William's behavior of avoiding a potential situation.