Define a mental disorder.

A mental disorder is defined as a condition that significantly disrupts an individual’s thinking, feeling, mood, ability to relate to others, and daily functioning. This definition encompasses a range of psychological issues that can hinder an individual's ability to navigate everyday life. Mental disorders are characterized not only by emotional distress but also by impairments in cognitive and behavioral functioning.

In this context, the other definitions do not capture the full spectrum of what constitutes a mental disorder. Emotional responses to life changes, as indicated in one of the options, can be a normal reaction and do not necessarily imply a disorder unless they become pervasive and impair functioning. A temporary feeling of anxiety or sadness points to common experiences that many individuals face without qualifying as a mental disorder. Lastly, describing a personality trait acknowledges differences in social interactions but does not consider the significant disruptions in functioning that are central to mental disorders. Therefore, the comprehensive nature of option A accurately reflects the complexity and severity involved in mental health conditions.