The problem described involves the expansion of subnet ranges at the branch office, which has caused connectivity issues due to the static routing configuration on the AWS Site-to-Site VPN connection. The solution needs to address the immediate issue with minimal administrative overhead for future expansions. Option A suggests determining a supernet for the branch office and replacing specific subnet routes with an aggregate route in the transit gateway route table. This approach simplifies the routing configuration by reducing the number of static routes and makes future expansions easier by allowing the inclusion of new subnets within the supernet without requiring additional route entries. Option B introduces AWS Direct Connect, which is not necessary for solving the immediate issue and adds complexity. Option C suggests switching to a dynamically routed VPN connection, which would solve the problem but requires more administrative effort to set up and manage. Option D involves creating a prefix list, which is a more complex solution than using a supernet and does not offer the same simplicity for future expansions. Therefore, Option A is the most efficient and least administratively burdensome solution for both the current issue and future expansions.