A young man is convinced that his phone is being monitored by spies from another country, even though he has no ties to the country and has never visited it. His conviction leads him to tell his friends and family not to contact him by phone. Which of the following symptoms is he exhibiting?

An obsession refers to persistent, unwanted thoughts or images that cause significant anxiety or distress. While the young man is preoccupied with the idea of being monitored, his primary symptom is not the anxiety from these thoughts but rather the firm belief in their truth, which aligns more closely with a delusion.

A hallucination involves experiencing sensations that seem real but are created by the mind, such as hearing voices or seeing things that aren't there. The young man is not perceiving any false sensory input; instead, he is firmly holding onto an irrational belief, which distinguishes his condition from a hallucination.

A compulsion is a repetitive behavior or mental act that an individual feels driven to perform in response to an obsession, often to reduce anxiety. The young man's actions of avoiding phone contact do not stem from an anxiety-driven behavior pattern but rather from his firmly held belief, which is indicative of a delusion.