Which of the following is a mental disease?

The correct option is C Neurosis

A mental disease causes abnormal changes in the behaviour and thoughts of a person because mind is affected. Neurosis is a mental illness in which person suffers from depression, obsessive behaviour, anxiety and feeling of unhappiness. The affected person shows worries and fears without any reason. The person remains in negative emotional state for a prolonged period of time.

So, the correct answer is 'Neurosis'.