The question revolves around the need for a scalable and efficient architecture that supports the addition of new Business Units (BUs) and Regions while ensuring isolation between BUs. Option A suggests creating a new transit gateway for each new BU in each Region and peering them with existing transit gateways. This approach, while feasible, could lead to operational complexity and inefficiency as the number of BUs and Regions grows. Option D proposes attaching new VPCs to the existing transit gateways and updating route tables to control traffic. This method does not inherently provide the required isolation between BUs and could complicate route management as the network scales. Options B and C both suggest using AWS Cloud WAN, which is designed to simplify the management of global networks. AWS Cloud WAN allows for the creation of segments, which can be used to logically isolate network traffic between different BUs. Option B mentions using segment actions to control traffic between segments, which provides a mechanism for fine-grained traffic control. However, Option C specifically mentions configuring the segments to isolate attachments, which directly addresses the requirement for isolating BUs from each other. Therefore, Option C is the most operationally efficient solution as it leverages AWS Cloud WAN's capabilities to meet the company's requirements for scalability, isolation, and operational efficiency.