Which of the following is a popular source for system hardening, with a compliance checker tool provided by the U.S. Department of Defense?

The correct choice reflects the role of Security Technical Implementation Guides (STIGs), which are comprehensive documents produced by the U.S. Department of Defense aimed specifically at enforcing security requirements for various government systems and technologies. STIGs provide detailed guidelines on how to configure and secure systems, ensuring they meet established security standards.

In addition to guidelines, STIGs often come with compliance checker tools that help organizations evaluate their systems against the recommended configurations and identify areas where security postures may be lacking. This tool aids in automating the compliance verification process, thus enhancing the overall effectiveness of system hardening efforts.

On the other hand, while the other options offer valuable security frameworks and guidelines, they do not specifically align with the context of a popular source for system hardening linked to a compliance checker tool provided by the U.S. Department of Defense. For instance, NIST 800-207 deals with zero trust architecture but does not focus directly on system hardening. The CIS Benchmark provides security best practices but is not specifically from the DoD. NIST 800-84 addresses guidelines for incident handling and not directly the hardening compliance aspects.