A solutions architect created a Cache VPC for an Amazon ElastiCache cluster and an App VPC for the application’s Amazon EC2 instances. Both VPCs are in the us-east-1 Region. The EC2 instances must access the ElastiCache cluster. Which solution meets these requirements MOST cost-effectively?

The most cost-effective solution is to create a VPC peering connection. VPC peering allows private IP address communication between two VPCs as if they were in the same network, without using public IPs or gateways. This is a direct, low-latency, and cost-effective method for connecting two VPCs within the same region. You must update route tables in both VPCs to direct traffic to the peered VPC and configure the ElastiCache security group to allow inbound connections from the application's security group. Using a Transit VPC (or AWS Transit Gateway) would also work but is generally more complex and expensive for connecting only two VPCs, as it's designed for hub-and-spoke connectivity with many VPCs. The incorrect options regarding security group configuration for the peering connection or Transit VPC are flawed because security groups are associated with instances/resources within a VPC, not with the peering connection itself or the Transit VPC as a whole for this specific traffic flow.