Which defense is NOT commonly used against a plaintiff's negligence suit?

The correct answer focuses on an approach that is not typically recognized as a valid defense in negligence cases. Unintentional negligence, while the term may imply that a defendant did not intend to cause harm and may refer to the nature of negligence itself, does not constitute a legally established defense. Legal defenses in negligence claims revolve around established principles that attribute some fault or responsibility to the plaintiff or clarify the circumstances of the incident.

In contrast, the other options represent recognized defenses in negligence cases. Assumption of risk involves a plaintiff knowingly accepting the potential risks associated with an activity, thereby limiting or eliminating the defendant's liability. Contributory negligence asserts that the plaintiff's own negligence contributed to their injury, which may reduce or eliminate damages. Comparative negligence allows for damages to be apportioned based on the degree of fault of each party involved. These defenses aim to address the shared blame or circumstances surrounding the incident, while unintentional negligence does not offer a structured legal argument, making it an outlier among common defenses.