Which designation should be applied to information that unauthorized disclosure could reasonably be expected to cause damage to national security?

The designation that should be applied to information whose unauthorized disclosure could reasonably be expected to cause damage to national security is "Confidential."

In the context of national security classification levels, "Confidential" is used to categorize information that could result in damage if disclosed. This classification serves as a protective measure to control access to sensitive information, ensuring that only individuals with the appropriate security clearance and need to know are granted access to it.

The other classifications, such as "Top Secret," are reserved for information that could cause exceptionally grave damage if disclosed, thus representing a higher level of sensitivity than "Confidential." The term "Restricted" is not a formal classification within the U.S. classification system, which typically includes "Confidential," "Secret," and "Top Secret." "Unauthorized disclosure" is a description of an action that may occur but does not represent a classification level itself. Therefore, "Confidential" is the correct choice as it aligns with established protocols regarding the protection of sensitive national security information.