What must a defendant prove to successfully use the "insanity defense"?

To successfully utilize the "insanity defense," a defendant must demonstrate that, due to a mental illness, they were unable to understand the nature of their actions or distinguish right from wrong at the time the crime was committed. This concept is rooted in the belief that individuals who are not in control of their mental faculties cannot be held fully accountable for their actions in the same way as those who are mentally sound. The standard often varies by jurisdiction, but generally, it hinges on the individual's mental state at the time of the offense, showcasing their incapacity to comprehend the consequences of their behavior.

The other options revolve around different legal concepts that do not pertain to the insanity defense. For instance, duress involves a person being forced to commit a crime under threat, self-defense relates to the justification of actions taken to protect oneself from harm, and intoxication may mitigate culpability but does not necessarily indicate a lack of understanding due to a mental health condition. Thus, the emphasis on a mental illness causing a lack of understanding of the act is what makes this answer the correct one in the context of the insanity defense.