A company uses transit gateways to route between its VPCs. Each transit gateway has one route table that contains attachments and routes for the VPCs in the same AWS Region. Each VPC route table currently includes routes to all other VPC CIDR blocks reachable through the transit gateways. Some VPCs route traffic to local NAT gateways. The company plans to create many new VPCs and needs the most operationally efficient method to add the new VPC CIDR ranges to every VPC route table. Which solution best satisfies this requirement?

Using a customer-managed prefix list is the most operationally efficient solution. You can define all VPC CIDR ranges in a single prefix list. When new VPCs are added, you only need to update this central prefix list. All VPC route tables configured to use this prefix list will automatically update, eliminating the need to manually modify each individual VPC route table. The other options are less efficient or incorrect. Propagating routes from the transit gateway to VPC route tables is not a native AWS feature. Using 0.0.0.0/10 as a destination would incorrectly route a large portion of the internet traffic through the transit gateway, bypassing local NAT gateways and potentially causing routing issues. Default route table association and propagation for transit gateway route tables manage routes within the transit gateway, not from the transit gateway to individual VPC route tables.