Which entities must coordinate seasonal planning and operations together?

The correct choice highlights the importance of coordination among distinct entities within the electric grid to ensure reliability and operational efficiency. Each Balancing Authority (BA) and Transmission Operator (TOP) is responsible for maintaining the balance of electricity supply and demand within their operational areas. However, electricity supply and infrastructure often extend across multiple jurisdictions and regions, necessitating collaboration with neighboring BAs and TOPs.

By working together, these entities can effectively coordinate seasonal planning, which involves anticipating changes in electricity demand and generation capacity due to seasonal variations. Additionally, coordination with the Reliability Coordinator (RC) is essential as the RC oversees the reliability of the bulk power system and helps facilitate communication and coordination among the BAs and TOPs.

This collaborative approach ensures that all potential factors affecting the reliability of the electric grid are considered, including fluctuations in renewable energy supply, which can vary seasonally, and the need to manage resources to prevent outages or system failures. The involvement of the RC adds an additional layer of oversight and strategic management, enhancing the grid's reliability across interconnected systems.

The other options do not address the necessary inter-entity cooperation that is critical for effective seasonal planning and operations at a broader scale, focusing instead on narrower or less relevant groupings.