Which entity provides implementing guidance for classified information?

The entity that provides implementing guidance for classified information is the ISOO Executive Office, which stands for the Information Security Oversight Office. This office operates under the National Archives and Records Administration and is responsible for overseeing the government's security classification system. It establishes policies and guidelines that ensure that classified information is appropriately marked, handled, and protected in accordance with federal laws and regulations.

The role of the ISOO is crucial in promoting best practices and standardizing procedures across various government agencies, ensuring that they comply with security protocols regarding classified data. This oversight function is vital for maintaining the integrity of national security while facilitating the appropriate sharing and access of information when necessary.

In contrast, while the Department of Homeland Security and the Federal Bureau of Investigation engage in various aspects of national security, their primary focus is not on providing implementing guidance for classified information. The National Archives and Records Administration does house ISOO within its framework, but the specific function of providing implementing guidance resides with ISOO itself. This distinguishes the ISOO as the correct answer in this context.