Which of the following is a key characteristic of mental illness?

The presence of hallucinations is a key characteristic of certain mental illnesses, particularly those classified within psychotic disorders, such as schizophrenia. Hallucinations can involve seeing, hearing, or feeling things that are not present in reality, significantly affecting a person's perception and experience of the world. This symptom often indicates a disruption in the individual's cognitive processes and can be a critical factor in diagnosing and treating mental health conditions.

In contrast, none of the other choices represent characteristics commonly associated with mental illness. Active cognitive engagement tends to be a sign of functioning well and does not align with confusion or disorganized thinking seen in some mental health disorders. Enhanced emotional stability generally indicates good mental health rather than illness, while improved rational thinking is similarly a trait of healthier cognitive functioning. Thus, the correct answer highlights a specific symptom that can indicate the presence of a mental health disorder.