Which of the following actions is necessary when a Cyber Security Incident is determined to be reportable?

When a Cyber Security Incident is determined to be reportable, notifying the Electricity Sector Information Sharing and Analysis Center (ES-ISAC) is essential. This body serves as a critical resource for utilities to share information regarding cybersecurity threats and incidents. By reporting the incident to ES-ISAC, organizations help foster a broad situational awareness within the electricity sector, which is vital for maintaining national security and protecting critical infrastructure. This communication allows other entities in the sector to be aware of potential threats and vulnerabilities, and it contributes to collective defense strategies.

The other options do not align with the required procedures for handling reportable incidents. For instance, notifying the press may lead to public panic or misinformation, which is not appropriate in an incident response context. Completing the response plan documentation is important but typically follows the reporting step; it's not the immediate necessary action upon determining a reportable incident. Implementing new software immediately can be hasty and could overlook necessary assessments or evaluations, leading to further vulnerabilities. Therefore, the clear focus on reporting to ES-ISAC underscores the importance of communication and information sharing within the industry during a cybersecurity incident.