Who must be a U.S citizen and either officers, enlisted persons E-6 or above, or civilians GS-6 or above and designated in writing?

The correct choice highlights a role that requires specific personnel qualifications, including U.S. citizenship and certain rank or civil service grade criteria. The position of an Assistant Security Manager involves sensitive responsibilities, often connected to maintaining security and managing classified information within a command.

Being a U.S. citizen is a requirement for positions that deal with national security or sensitive information to ensure loyalty and adherence to U.S. laws. Additionally, the requirement for this role to be filled by a member of the military (E-6 or above) or a civilian (GS-6 or above) ensures that the individuals in these roles have reached a level of experience and responsibility that correlates with the security challenges they may face.

In contrast, while other positions like Commanding Officer or Chief of Naval Operations may also require U.S. citizenship, they encompass broader command responsibilities that are not solely dedicated to security management. The Directives Record Keeper also does not directly correlate with the combination of personnel criteria specified in the question. Thus, the Assistant Security Manager is the most aligned with the stated requirements of citizenship and specified ranks or grades.