Final answer:
The term for the high level of co-occurrence of multiple mental disorders is a)comorbidity.
Step-by-step explanation:
The troublesome aspect of diagnosing and characterizing mental illness is known as comorbidity. Comorbidity refers to the high level of co-occurrence of multiple disorders in individuals with one disorder. This means that people with one mental disorder tend to have other mental disorders as well. For example, a person with depression may also have anxiety or substance abuse issues. The concept of comorbidity highlights the complex nature of mental illness and the need for comprehensive assessment and treatment.