Which of the following is the focus of Article 820?

Article 820 of the National Electrical Code (NEC) specifically addresses community antenna television and radio distribution systems. This article provides the necessary regulations and guidelines for the installation and maintenance of coaxial cables and other wiring associated with these types of systems, ensuring safety and reliability in the distribution of television and radio signals.

The focus on community antenna systems is crucial as it helps ensure that installation practices align with safety standards to avoid hazards such as fire or electrical shock that could arise from improper installations. Further, it also encompasses grounding, bonding, and associated equipment, which are vital to the efficacy and longevity of the distribution systems.

Other topics mentioned do not relate directly to Article 820, demonstrating why the correct answer emphasizes community antenna television and radio systems over concepts like network optimization, data communication standards, or residential wiring.