Mental disorders may be determined by symptoms. Which of the following is a symptom?

The correct answer encompasses all the listed options, as they each represent symptoms that could indicate a mental disorder.

Sudden changes in behavior can signal various psychological issues. For instance, if someone who is typically outgoing suddenly becomes withdrawn, this might be a sign of underlying mental health problems that need further exploration. Behavioral changes can often serve as early indicators of emotional distress or psychiatric conditions.

Behavior that is not harmonious with a situation reflects a disconnect between a person's response and the context of that response. For example, a reaction of extreme anxiety in a non-threatening environment could highlight a potential mental disorder, suggesting that the individual is struggling with emotional regulation.

Unduly prolonged depression is a classic symptom associated with mood disorders, such as major depressive disorder. When depression persists longer than typical or manageable periods, it becomes a concern and can significantly affect a person's daily function and quality of life, indicating a mental health condition that may require intervention.

By recognizing that each of these aspects can signify a deeper issue, the response effectively demonstrates a comprehensive understanding of symptoms related to mental disorders, thus validating the choice that all are indeed symptoms.