Which of the following is a justification defense?

Self-defense is classified as a justification defense because it argues that a person's actions were necessary to prevent imminent harm to themselves or others. In this context, the law recognizes that sometimes individuals must take actions that would normally be considered criminal in order to protect themselves from a greater evil. The underlying principle is that the act of defending oneself is justified when faced with an immediate threat, making it a socially acceptable response under those circumstances.

The other concepts mentioned, like duress and insanity, relate to different types of defenses. Duress involves acting under pressure or threat of harm from another, which negates voluntary action but does not justify the act. Insanity refers to a mental state that impairs a person's ability to understand the nature of their actions or distinguish right from wrong, which means that while it may excuse behavior, it does not justify it in the context of criminal responsibility. Coercion is similar to duress, emphasizing the compulsion aspect rather than a justification for the action taken. Thus, self-defense stands out as the only justification defense among the choices listed.