Which of the following is indicative of underlying mental illnesses?

Paranoia is indicative of underlying mental illnesses as it often suggests a distorted perception of reality that can arise from conditions such as schizophrenia, paranoid personality disorder, or severe anxiety disorders. Individuals experiencing paranoia may exhibit irrational fears or beliefs that others are plotting against them, which can significantly impair their ability to function in daily life and maintain relationships.

The other options, while they may be related to mental health issues, do not point as directly to specific underlying mental illnesses. Chronic fatigue can stem from various physical or psychological conditions, excessive optimism may reflect a generally positive attitude and is not typically linked with mental disorders, and social withdrawal can be a symptom of multiple issues but does not specify a mental illness in the way paranoia does.