Expanding the CIDR range of the relevant subnet is the correct solution because it allows for more IP addresses to become available within the subnet, enabling the clusters to grow by adding more nodes as needed. This approach directly addresses the issue of IP address exhaustion without the need for creating new subnets or VPCs, which would not inherently solve the IP availability problem in the existing subnet. The first two and last two IP addresses of a primary IP address range are reserved by Google Cloud, but expanding the range ensures scalability. Other options like adding an alias IP range or creating new subnets or VPCs do not effectively expand the available IP space for the existing clusters' growth.