Which of the following is NOT one of the systems surrounding police in Homeland Security?

The public transportation system is typically considered an essential infrastructure element but does not directly pertain to the systems specifically surrounding police in the context of Homeland Security. While police may have roles in ensuring safety and security within public transportation, the system itself is not inherently a law enforcement or security domain.

On the other hand, the national security system, criminal justice system, and security system are all interrelated to law enforcement operations within Homeland Security. The national security system involves government actions aimed at protecting the state and its citizens, which directly aligns with policing efforts. The criminal justice system encompasses the legal framework that law enforcement operates within, while security systems often refer to measures and protocols used to maintain safety, including those implemented by police agencies. Therefore, the public transportation system stands apart from these direct law enforcement-related frameworks.